1 Developmental Genetics Program, Department of Cell Biology, The Skirball
Institute and Howard Hughes Medical Institute, NYU School of Medicine, New
York, NY 10016, USA
2 Cell Biology and Metabolism Branch, National Institutes of Child Health and
Human Development, National Institutes of Health, Bethesda, MD 20892,
USA
Author for correspondence (e-mail:
lehmann{at}saturn.med.nyu.edu)
Accepted 30 December 2004
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Drosophila, twin, CCR4, Oogenesis, CycA, Bam
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
During Drosophila oogenesis, the cell cycle is tightly regulated
to coordinate egg chamber production. At the beginning of oogenesis, in the
germarium, cystoblasts undergo four rounds of synchronous, incomplete cell
divisions to generate cysts of 16 germ cells interconnected by ring canals.
Two cells, the pro-oocytes, share ring canal connections with four neighbors.
One of these cells, the oocyte, arrests in Prophase I of meiosis and condenses
its chromosomes into a spherical karyosome. The other cells all become
polyploid nurse cells and produce mRNA and proteins, such as Orb, that are
trafficked to the oocyte (Spradling,
1993). Elevation of CycA and of CycE can induce the cyst to
undergo a fifth division (Doronkin et al.,
2003
; Lilly et al.,
2000
; Ohlmeyer and Schupbach,
2003
). CycE/Cdk2 kinase activity also directs germ cell fate.
Increasing CycE/Cdk2 kinase activity by removing the Cdk-inhibitor
dacapo directs oocytes to take on nurse cell characteristics. A
mutation that reduces CycE/Cdk2 kinase activity levels results in more cells
with oocyte character (Hong et al.,
2003
; Lilly and Spradling,
1996
). Study of bag of marbles (bam) further
illustrates the close relationship between cell cycle control and
differentiation in the female germ line. Consistent with its role in germ cell
differentiation, loss of bam results in uncontrolled stem cell
proliferation without differentiation
(McKearin and Ohlstein, 1995
),
and ectopic bam causes stem cell loss
(Ohlstein and McKearin, 1997
).
A role for Bam in cell cycle control is revealed by the finding that reduction
of bam gene copy number suppresses the extra cyst division induced by
excess cyclin activity (Lilly et al.,
2000
).
The fusome, a vesicular organelle that contains cytoskeletal proteins such
as Hu Li Tai Shao (Hts), coordinates cell cycle and cell fate regulation in
the cysts. With each cyst division, the fusome branches and extends through
the ring canals connecting new daughter cells to the cyst
(de Cuevas et al., 1996;
Lin et al., 1994
). One of the
daughters always inherits more fusome material than the others and this cell
has been posited to differentiate as the oocyte
(de Cuevas and Spradling, 1998
;
Deng and Lin, 2001
;
Lin and Spradling, 1995
;
McKearin, 1997
). During the
cyst divisions, the fusome associates with one pole of each mitotic spindle to
regulate the division plane and the number and synchrony of divisions
(de Cuevas et al., 1997
;
McKearin, 1997
). Loss of Hts
eliminates the fusome and disrupts synchrony and number of the cyst divisions
as well as oocyte specification (Lin et
al., 1994
; Yue and Spradling,
1992
; Zaccai and Lipshitz,
1996
). CycA colocalizes with the fusome during the M-phase of
cystoblast divisions (Lilly et al.,
2000
) and may help the fusome to coordinate the number and timing
of divisions. Components of the proteasome are also localized to the fusome,
where they may play a role in CycE turnover
(Doronkin et al., 2003
;
Ohlmeyer and Schupbach,
2003
).
In this study, we describe the molecular identification and phenotypic
characterization of twin, a gene required for synchronous cyst
divisions, oocyte fate specification and cyst maturation. We show that
twin encodes the Drosophila homolog of the yeast
ccr4 gene. ccr4 (carbon-catabolite-repression) was first
identified in S. cerevisiae as a regulator of RNA levels of the
alcohol-dehydrogenase II gene (Denis,
1984). Although CCR4 protein was previously shown to associate
with basal transcription machinery (Liu et
al., 1998
), recent data demonstrate that CCR4 catalyzes the
degradation of poly(A) tails in yeast and flies
(Chen et al., 2002
;
Tucker et al., 2002
;
Temme et al., 2004
). We show
that in twin germaria, CycA is misexpressed and cycA mRNA
has a longer poly(A) tail. Furthermore, decreasing the dose of cycA
suppresses twin egg chamber degradation. We also find that
twin germaria accumulate less cytoplasmic Bam protein. Genetically
increasing the dose of bam suppresses twin cyst division and
oocyte fate specification defects. We conclude that Twin/Ccr4 is required for
the synchrony and number of cyst divisions and the oocyte/nurse cell fate
decision. We propose that Twin/Ccr4 exerts control via regulation of poly(A)
tail length of mitotic cyclins and by indirectly affecting the expression of
others factors required for germline differentiation, such as Bam.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Comparison and quantitation of twin phenotypes
Counts are from stages 3-10. We did not count chambers within the germaria
because, especially in twin germaria, it is difficult to determine
number and fate of cells in immature chambers. All crosses were kept at
25°C except for the initial quantitation of the twin phenotypes
(Fig. 1) which were at room
temperature to reveal the full range of phenotypes.
|
P-insertion site identification
Genomic DNA was digested with BamHI and BglII. The DNA
was then ligated and PCR was performed with the Expand Hi Fidelity kit using
primers extending outward from the P element ends. Sequencing ABI sequencing
of the fragments and analysis with Editview, DNAstar (Lasergene) and BLAST
(Altschul et al., 1997)
followed.
Northern analysis
RNA was isolated from twinry3 homozygous and wild-type
ovaries using Trizol. The RNA was then poly(A) purified using the PolyATract
mRNA Isolation System III (Promega), and was run out on an agarose gel,
blotted and probed using standard methods.
Mutagenesis, screening, allele sequencing and sequence analysis
We mutagenized ru h th st sr e ca males with EMS. Mutants were
tested for failure to complement the ry5 allele for female sterility.
ry5 is viable in trans to deficiency and fertile in males, so null
alleles can be isolated in such a screen and recovered via male progeny. ESTs
were ordered from the Berkeley Drosophila Genome Project and were
ABI-sequenced from ends. New primers were generated from each round of
sequencing to generate the next round.
PAT assays
As described by Salles and Strickland
(Salles and Strickland, 1999).
RNA was prepared from ovary extracts of young (1- and 2-day-old) adults using
TRIZOL. Very young twin and wild-type adults had comparably little
development of mid- and late-stage egg chambers. Total RNA (100 ng) used for
first-strand synthesis. PCR reactions performed with expand Hi-Fidelity kit.
32P dCTP was added to 10 mM cold dNTP stock before setting up PCR
reactions, and a fraction of the PCR product was tested on a gel to permit
approximately equal loading of hot PCR product for the experiment. Samples
were run on 6% acrylamide gels, dried down and exposed as autoradiographs.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
When egg chambers contained oocytes, these oocytes appeared normal and contained the most ring canals. We observed chambers containing three germ cells, including an oocyte (not shown) as well as rare chambers specifying two oocytes (Fig. 1F). In all cases, the oocytes condensed their chromosomes into a normal karyosome and accumulated oocyte specific factors such as Orb protein (Fig. 1C) and osk mRNA (not shown). We observed two oocytes in chambers containing too many, too few or exactly 16 germ cells. In these cases, both oocytes appeared normal, even to the extent that they attracted migrating follicle cells in stage 9 egg chambers. When there were two oocytes they were often both at the posterior of the chamber (Fig. 1F), although they were also found at opposite ends of the chamber (not shown). Because cyst division and oocyte specification defects occur independently in twin mutants, Twin may couple these processes in the wild type to ensure exactly one oocyte forms in each 16 cell egg chamber.
In spite of the striking defects observed in twin ovaries, most other tissues in twin animals are apparently unaffected. twin/Df flies are viable and males are fertile. Within the ovary, the defect is restricted to the germline. Follicle cells form normal epithelia and migrate properly. In germline clones, twin mutant germ cells exhibit mutant phenotypes in an otherwise heterozygous background (data not shown), indicating that the functioning of twin is required in the germline.
twin encodes a homolog of yeast CCR4, a deadenylase
To determine how twin affects germline development, we molecularly
identified the gene using two P-element insertion alleles,
twinry3 and twinry5
(Spradling, 1993). We reverted
the mutant phenotype by precise excision of the P-elements, confirming that
the insertions caused the twin defects. twin maps to
chromosomal region 95F and the twin alleles failed to complement
Df(3R)CRB87-4. Inverse PCR and sequencing of the distal and proximal
genomic DNA flanking the P-element insertions showed that the
twinry3 and twinry5 transposons were
only offset by 1 bp from each other and in opposite orientations. Using BLAST
analysis we found ESTs that matched the distal flanking region
(Fig. 2A,B). These ESTs
included several splice isoforms of a Drosophila ccr4 homolog that
differ in their 5'UTRs but not in their coding regions
(Fig. 2B). Sequence analysis
showed that the P-insertions as well as the P-element proximal DNA fell in a
15 kb intron of the ccr4 gene.
|
We next wanted to see if ccr4 is expressed in a pattern consistent
with the role of twin in oogenesis. Consistent with FlyBase reports
we find that ccr4 RNA is expressed throughout embryogenesis. Newly
deposited embryos show very strong maternal deposition of ccr4 RNA
(Fig. 2D). The maternal
transcript is rapidly degraded, but the germline and soma exhibit low-level
uniform expression in the syncytial blastoderm
(Fig. 2E) and throughout
embryogenesis (Fig. 2F,G).
ccr4 is expressed throughout oogenesis, most strongly in the germline
but also in the soma (Fig.
2I,J,K) (see Temme et al.,
2004).
S. cerevisiae CCR4 has three functional domains
(Fig. 3A)
(Malvar et al., 1992). The
N-terminal region is rich in glutamine and asparagine. The central region
contains five leucine-rich repeats (LRR), which are crucial for dimerization
with CAF1/POP1, an adapter protein that facilitates the association of CCR4
with other proteins and is required for deadenylation of targets
(Clark et al., 2004
). The Twin
LRR domain contains three and a half repeats
(Fig. 3A,B) which are most
similar to the repeats shown to be essential to yeast CCR4 function
(Clark et al., 2004
). The
C-terminal domain of S. cerevisiae CCR4 has 3'-5'
exoribonuclease and ssDNA exonuclease activity, and it catalyses the
poly(A)-deadenylation reaction (Tucker et
al., 2002
; Tucker et al.,
2001
; Chen et al.,
2002
; Draper et al.,
1994
; Malvar et al.,
1992
). The catalytic domain of Twin shares extensive homology with
S. cerevisiae CCR4. Blast analysis of the Drosophila genome
showed two other predicted ccr4 homologs in Drosophila,
angel and Dnocturnin (Fig.
3A). Angel (Kurzik-Dumke and
Zengerle, 1996
) and Dnocturnin share extensive homology with the
CCR4 catalytic domain, although Dnocturnin is missing an asparagine shown to
be essential for catalysis (Chen et al.,
2002
). Neither protein possesses an LRR repeat region
(Fig. 3A); Twin is therefore
the best candidate to encode the Drosophila CCR4 homolog. Consistent
with a prominent role of Twin in deadenylation, Temme et al.
(Temme et al., 2004
) showed
that Drosophila CCR4 catalyzes deadenylation when purified from
extracts. Partial depletion of CCR4 either from tissue culture or flies led to
an overall increase in bulk poly(A)tail length and more specifically affected
the poly(A)tail length and stability of heat-shock, hsp 70, RNA.
|
Misregulation of mitotic cyclins contributes to twin defects
twin mutants exhibit defects in the divisions of the cystoblast.
Such defects have been observed in mutants directly affecting cell cycle
regulators (Mata et al., 2000)
and in mutants affecting the integrity of the fusome, which co-localizes with
CycA in a cell cycle-dependent manner (de
Cuevas et al., 1996
; Lilly et
al., 2000
; Lin et al.,
1994
). Since CCR4 in S. cerevisiae regulates gene
expression, we examined twin mutants for changes in cyclin regulation
in the germarium (Fig. 4A-F).
Staining of wild-type or twinS1/+ germaria showed that, as
reported previously, CycA is expressed in the dividing cysts of germarium
region 1 and is downregulated in the post-mitotic cysts from region 2 on
(Fig. 4A,C). In
twinS1/Df ovaries, CycA was ectopically expressed
throughout the germarium, including what should be the postmitotic regions
(Fig. 4B). Similar, but less
strong misexpression was also observed in twinry3/Df
(Fig. 4D). Somatic
expression appeared unaffected. We also analyzed the expression of CycB, which
acts together with CycA in promoting entry into mitosis
(Knoblich and Lehner, 1993
;
Knoblich et al., 1994
). CycB
is misexpressed in a pattern similar to that of CycA in twin germaria
(Fig. 4E,F), although in
contrast to the very consistent CycA misexpression phenotype, not all germaria
misexpressed CycB. CycB misexpression may be an indirect result of reduced
Twin activity or CycB may have other important regulators in addition to Twin.
We stained ovaries with antibodies raised against the S-phase cyclin CycE, and
did not detect a difference between twinS1/Df and
twin/+ germaria (not shown). Our findings suggest that Twin is
required for downregulating CycA expression in the germarium and, to a lesser
extent, expression of CycB.
|
Our findings show that CycA levels are elevated in twin. We analyzed whether reduction in the levels of cyclins affected the twin phenotypes. For these experiments we used the twinry3/Df allelic combination, whose intermediate phenotype seemed suitable for modification studies. Removing one copy of cycA markedly suppressed the number of stage 3-10 chambers that were degrading (Table 2, 51% compared with 70%, a relative suppression of 30%, P<0.001), suggesting that excess CycA induces chamber degradation in twin (see Discussion, Table 2). We did not observe a significant change in the twinry3/Df phenotype when we removed one copy of cycE or cycB (Table 2). It remains unclear whether a specific stage of the cell cycle is particularly sensitive to loss of Twin function. Staining germaria with anti-phospho-Histone 3 antiserum did not reveal an enrichment of cysts with the condensed chromosomes characteristic of early to mid-M phase (not shown), but twin cysts could be delayed elsewhere in the cell cycle. On the basis of the CycA expression and genetic interaction experiments, we conclude that misexpression of CYCA contributes to the twin egg chamber phenotype.
|
We stained twin and wild-type ovaries with BamC antiserum
(Fig. 5). In wild-type ovaries,
BamC stains the cystoblasts and the dividing cysts but not later stages
(McKearin and Ohlstein, 1995)
(Fig. 5A,A'). The BamC
staining in the dividing cysts of twinS1/Df ovaries was
much fainter (Fig.
5B,B'). Most germaria showed quite weak or undetectable
levels of BamC, although some germaria exhibited strong BamC expression. We
conclude that Bam is downregulated or not localized properly to the cytoplasm
(and thus not detectable by the BamC antiserum) in twin mutants.
|
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Our twin alleles are viable and specifically affect the female
germline. In S. cerevesiae, ccr4 mutations are not lethal, although
CCR4 is thought to be the main cytoplasmic deadenylase
(Denis, 1984). It is possible
that angel and Dnocturnin (CG4796), two other genes with
extensive homology to the ccr4 catalytic domain but lacking the
crucial LRR repeats, can partially compensate for loss of Twin function.
Alternatively, as our mutations are probably not complete nulls, oogenesis may
be more sensitive than the soma to decreased Twin function. Like the ovary,
the early embryo relies on precise post-transcriptional gene regulation
(Tadros et al., 2003
). The
mature egg contains high levels of maternally loaded twin, consistent
with a role for Twin in deadenylation, and probably explaining why
twin mutants carry out embryogenesis normally.
Control of cyclins in development
Mitotic cells regulate cyclin levels in order to progress through the cell
cycle. At the protein level, Drosophila regulates CycA, CycB and
CycE, via proteasome-mediated degradation
(Doronkin et al., 2003;
Echard and O'Farrell, 2003
;
Sigrist et al., 1995
). In the
Drosophila ovary, the novel protein Encore has been proposed to
localize components of the proteasome complex to the fusome to regulate CycE
(Doronkin et al., 2003
;
Ohlmeyer and Schupbach, 2003
).
encore mutant cysts undergo an extra cell division and contain 32
cells (Hawkins et al., 1996
),
probably as a consequence of misexpressing not only CycE, but also CycA
(Ohlmeyer and Schupbach,
2003
). Other experiments have shown that cyst divisions are
sensitive to CycA levels. Adding a brief pulse of CycA by inducing a
heat-shock construct can lead to an extra round of cyst division, suggesting
that downregulation of CycA is crucial for cell cycle progression
(Lilly et al., 2000
). Only a
small number of cysts respond to such a CycA pulse, suggesting that in the
wild type not all germ cells are in a susceptible phase of the cell cycle (G2)
during which they can respond to CycA.
Cyclin RNA levels are regulated by control of poly(A) tail length. In
Xenopus and mouse oocytes, cycB RNA is not translated in the
absence of CPEB-mediated poly(A) tail lengthening. Longer poly(A) tails also
enhance cyclin translation in Drosophila embryos. In the
Drosophila ovary, Orb, the CPEB homolog, regulates poly(A) tail
length and expression of its own RNA and oskar RNA
(Castagnetti and Ephrussi,
2003; Tan et al.,
2001
). Consistent with a role for Orb in cyclin regulation and
cyst division, orb mutant cysts frequently contain eight germ cells
(Lantz et al., 1994
).
Our data suggest that Twin-mediated deadenylation of cyclin RNA regulates
cyst divisions. Cyclin polyadenylation has been well studied, but much less is
known about cyclin RNA deadenylation. In Drosophila, Nanos and
Pumilio have been shown to control deadenylation of cycB mRNA in
primordial germ cells (Asaoka-Taguchi et
al., 1999). Furthermore, Xenopus Pumilio interacts with
CPEB (Richter and Theurkauf,
2001
), and Nanos, Pumilio and Orb/CPEB are all expressed early in
Drosophila oogenesis (Forbes and
Lehmann, 1998
). It is intriguing to speculate that Twin may
regulate the poly(A) tail lengths in the dividing cyst in conjunction with
Nanos, Pumilio and/or Orb.
Twin and Bam
BamC expression is reduced in twin germaria; a phenotype we would
not predict if Twin directly regulated Bam expression via deadenylation.
Indeed, we did not observe a substantial change in bam poly(A) tail
length in twin ovaries. We therefore propose that bam is an
indirect target of Twin/Ccr4.
Although bam is known to control the differentiation of the cystoblast and to promote cyst division, the biochemical role of Bam is unknown. Removing one copy of bam suppresses the extra division in cysts lacking encore or overexpressing CycA. Our results further implicate Bam in the events of early oogenesis. Increased bam expression suppresses not only the cyst division defects observed in twin mutants, but also the twin oocyte specification defects. Because Twin regulates cycA directly and may regulate Bam indirectly, the simplest model would posit that high levels of CycA are sufficient to suppress Bam expression. Two pieces of evidence argue against this model: Bam and CycA are both present at high levels in the dividing cyst; and Bam is required for the fifth cyst division induced by high levels of CycA. In addition, hs-bam induces stem cells to develop into normal cysts, indicating that high Bam levels do not disrupt CycA expression. We favor a model by which Bam and CycA act in parallel to each other, downstream of Twin.
Model for twin function in the ovary
Although several models could explain our data, we propose that increased
mitotic cyclin levels together with low Bam expression cause many of the
twin phenotypes. If Bam expression were normal, overexpressing
cyclins could lead to extra cyst divisions. The low level of Bam in
twin germaria does not permit continued cell division, yet cyclin
levels remain high, delaying cell cycle progression and probably causing the
egg chamber degradation we observe in twin. This model is consistent
with the fact that reducing the copy number of bam suppresses the
extra cyst division phenotype of encore and of hs-cycA
(Hawkins et al., 1996;
Lilly et al., 2000
).
Corroborating evidence comes from the observation that reducing the gene dose
of cycA or increasing the dose of bam can partially suppress
the degradation phenotype. However, there are likely to be other, unidentified
targets of twin that also contribute to the twin
phenotype.
twin and hts mutants disrupt the number and synchrony of
cyst divisions and oocyte specification. This array of defects is not shared
by the cell cycle mutants discussed above or by other mutants such as
orb (Lantz et al.,
1994), the M-phase inhibitor tribbles or the M-phase
activator string (Mata et al.,
2000
), which affect the number but not the synchrony of cyst
divisions. Comparison of twin and hts may therefore be
instructive. hts cysts have no fusome, and are thought consequently
not to coordinate the cyst divisions (Lin
et al., 1994
; Yue and
Spradling, 1992
). By contrast, cysts in twin mutants
contain branched fusomes that are capable of colocalizing with CycA,
suggesting the possibility that Twin/Ccr4 gene regulation may mediate the
coordination of the cyst divisions with oocyte specification downstream of the
fusome.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
Footnotes |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Altschul, S. F., Madden, T. L., Schaffer, A. A., Zhang, J.,
Zhang, Z., Miller, W. and Lipman, D. J. (1997). Gapped
BLAST and PSI-BLAST: a new generation of protein database search programs.
Nucleic Acids Res. 25,3389
-3402.
Asaoka-Taguchi, M., Yamada, M., Nakamura, A., Hanyu, K. and Kobayashi, S. (1999). Maternal Pumilio acts together with Nanos in germline development in Drosophila embryos. Nat. Cell Biol. 1,431 -437.[CrossRef][Medline]
Castagnetti, S. and Ephrussi, A. (2003). Orb
and a long poly(A) tail are required for efficient oskar translation at the
posterior pole of the Drosophila oocyte. Development
130,835
-843.
Chen, J., Rappsilber, J., Chiang, Y. C., Russell, P., Mann, M. and Denis, C. L. (2001). Purification and characterization of the 1.0 MDa CCR4-NOT complex identifies two novel components of the complex. J. Mol. Biol. 314,683 -694.[CrossRef][Medline]
Chen, J., Chiang, Y. C. and Denis, C. L.
(2002). CCR4, a 3'-5' poly(A) RNA and ssDNA
exonuclease, is the catalytic component of the cytoplasmic deadenylase.
EMBO J. 21,1414
-1426.
Clark, L., Viswanathan, P., Quigley, G., Chiang, Y., McMahon,
J., Yao, G., Chen, J., Nelsbach, A. and Denis, C.
(2004). Systematic mutagenesis of the leucine-rich repeat (LRR)
domain of CCR4 reveals specific sites for binding to CAF1 and a separate
critical role for the LRR in CCR4 deadenylase activity. J. Biol.
Chem. 279,13616
-13623.
de Cuevas, M., Lilly, M. A. and Spradling, A. C. (1997). Germline cyst formation in Drosophila. Annu. Rev. Genet. 31,405 -428.[CrossRef][Medline]
de Cuevas, M., Lee, J. K. and Spradling, A. C.
(1996). alpha-spectrin is required for germline cell division and
differentiation in the Drosophila ovary. Development
122,3959
-3968.
de Cuevas, M. and Spradling, A. (1998).
Morphogenesis of the Drosophila fusome and its implications for oocyte
specification. Development
125,2781
-2789.
Deng, W. and Lin, H. (2001). Asymmetric germ cell division and oocyte determination during Drosophila oogenesis. Int. Rev. Cytol. 203,93 -138.[Medline]
Denis, C. L. (1984). Identification of new
genes involved in the regulation of yeast alcohol dehydrogenase II.
Genetics 108,833
-844.
Denis, C. L. and Malvar, T. (1990). The CCR4
gene from Saccharomyces cerevisiae is required for both nonfermentative and
spt-mediated gene expression. Genetics
124,283
-291.
Doronkin, S., Djagaeva, I. and Beckendorf, S. K. (2003). The COP9 signalosome promotes degradation of Cyclin E during early Drosophila oogenesis. Dev. Cell 4, 699-710.[CrossRef][Medline]
Draper, M. P., Liu, H. Y., Nelsbach, A. H., Mosley, S. P. and Denis, C. L. (1994). CCR4 is a glucose-regulated transcription factor whose leucine-rich repeat binds several proteins important for placing CCR4 in its proper promoter context. Mol. Cell. Biol. 14,4522 -4531.[Abstract]
Duronio, R. J., Brook, A., Dyson, N. and O'Farrell, P. H. (1996). E2F-induced S phase requires cyclin E. Genes Dev. 10,2505 -2513.[Abstract]
Echard, A. and O'Farrell, P. H. (2003). The degradation of two mitotic cyclins contributes to the timing of cytokinesis. Curr. Biol. 13,373 -383.[CrossRef][Medline]
Ephrussi, A., Dickinson, L. K. and Lehmann, R. (1991). Oskar organizes the germ plasm and directs localization of the posterior determinant nanos. Cell 66, 37-50.[CrossRef][Medline]
Forbes, A. and Lehmann, R. (1998). Nanos and
Pumilio have critical roles in the development and function of Drosophila
germline stem cells. Development
125,679
-690.
Fuchimoto, D., Mizukoshi, A., Schultz, R. M., Sakai, S. and
Aoki, F. (2001). Posttranscriptional regulation of cyclin A1
and cyclin A2 during mouse oocyte meiotic maturation and preimplantation
development. Biol. Reprod.
65,986
-993.
Gilboa, L. and Lehmann, R. (2004). Repression of primordial germ cell differentiation parallels germ line stem cell maintenance. Curr. Biol. 14,981 -986.[CrossRef][Medline]
Groisman, I., Jung, M. Y., Sarkissian, M., Cao, Q. and Richter, J. D. (2002). Translational control of the embryonic cell cycle. Cell 109,473 -483.[Medline]
Hawkins, N. C., Thorpe, J. and Schupbach, T.
(1996). Encore, a gene required for the regulation of germ line
mitosis and oocyte differentiation during Drosophila oogenesis.
Development 122,281
-290.
Hong, A., Lee-Kong, S., Iida, T., Sugimura, I. and Lilly, M.
A. (2003). The p27cip/kip ortholog dacapo maintains the
Drosophila oocyte in prophase of meiosis I.
Development 130,1235
-1242.
Johnston, L. A. (2000). The trouble with tribbles. Curr. Biol. 10,R502 -R504.[CrossRef][Medline]
Kai, T. and Spradling, A. (2004). Differentiating germ cells can revert into functional stem cells in Drosophila melanogaster ovaries. Nature 428,564 -569.[CrossRef][Medline]
Knoblich, J. A. and Lehner, C. F. (1993). Synergistic action of Drosophila cyclins A and B during the G2-M transition. EMBO J. 12,65 -74.[Abstract]
Knoblich, J. A., Sauer, K., Jones, L., Richardson, H., Saint, R. and Lehner, C. F. (1994). Cyclin E controls S phase progression and its down-regulation during Drosophila embryogenesis is required for the arrest of cell proliferation. Cell 77,107 -120.[Medline]
Kurzik-Dumke, U. and Zengerle, A. (1996). Identification of a novel Drosophila melanogaster gene, angel, a member of a nested gene cluster at locus 59F4,5. Biochim. Biophys. Acta 1308,177 -181.[Medline]
Lantz, V., Chang, J. S., Horabin, J. I., Bopp, D. and Schedl, P. (1994). The Drosophila orb RNA-binding protein is required for the formation of the egg chamber and establishment of polarity. Genes Dev. 8,598 -613.[Abstract]
Lee, L. and Orr-Weaver, T. (2003). Regulation of cell cycles in Drosophila development: intrinsic and extrinsic cues. Annu. Rev. Genet. 37,545 -578.[CrossRef][Medline]
Lehmann, R. and Tautz, D. (1994). In situ hybridization to RNA. Methods Cell Biol. 44,575 -598.[Medline]
Lilly, M. A. and Spradling, A. C. (1996). The Drosophila endocycle is controlled by Cyclin E and lacks a checkpoint ensuring S-phase completion. Genes Dev. 10,2514 -2526.[Abstract]
Lilly, M. A., de Cuevas, M. and Spradling, A. C. (2000). Cyclin A associates with the fusome during germline cyst formation in the Drosophila ovary. Dev. Biol. 218, 53-63.[CrossRef][Medline]
Lin, H. and Spradling, A. C. (1995). Fusome asymmetry and oocyte determination in Drosophila. Dev. Genet. 16,6 -12.[Medline]
Lin, H., Yue, L. and Spradling, A. C. (1994).
The Drosophila fusome, a germline-specific organelle, contains membrane
skeletal proteins and functions in cyst formation.
Development 120,947
-956.
Liu, H. Y., Badarinarayana, V., Audino, D. C., Rappsilber, J.,
Mann, M. and Denis, C. L. (1998). The NOT proteins are part
of the CCR4 transcriptional complex and affect gene expression both positively
and negatively. EMBO J.
17,1096
-1106.
Malvar, T., Biron, R. W., Kaback, D. B. and Denis, C. L.
(1992). The CCR4 protein from Saccharomyces cerevisiae contains a
leucine-rich repeat region which is required for its control of ADH2 gene
expression. Genetics
132,951
-962.
Mata, J., Curado, S., Ephrussi, A. and Rorth, P. (2000). Tribbles coordinates mitosis and morphogenesis in Drosophila by regulating string/CDC25 proteolysis. Cell 101,511 -522.[Medline]
McKearin, D. (1997). The Drosophila fusome, organelle biogenesis and germ cell differentiation: if you build it. BioEssays 19,147 -152.[Medline]
McKearin, D. and Christerson, L. (1994). Molecular genetics of the early stages of germ cell differentiation during Drosophila oogenesis. Ciba Found. Symp. 182,210 -219.[Medline]
McKearin, D. and Ohlstein, B. (1995). A role
for the Drosophila bag-of-marbles protein in the differentiation of
cystoblasts from germline stem cells. Development
121,2937
-2947.
Morris, J. Z., Navarro, C. and Lehmann, R.
(2003). Identification and analysis of mutations in bob,
Doa and eight new genes required for oocyte specification and development
in Drosophila melanogaster. Genetics
164,1435
-1446.
Ohlmeyer, J. and Schupbach, T. (2003). Encore
facilitates SCF-Ubiquitin-proteasome-dependent proteolysis during Drosophila
oogenesis. Development
130,6339
-6349.
Ohlstein, B. and McKearin, D. (1997). Ectopic
expression of the Drosophila Bam protein eliminates oogenic germline stem
cells. Development 124,3651
-3662.
Richter, J. D. and Theurkauf, W. E. (2001).
Development. The message is in the translation.
Science 293,60
-62.
Salles, F. J. and Strickland, S. (1999). Analysis of poly(A) tail lengths by PCR: the PAT assay. Methods Mol. Biol. 118,441 -448.[Medline]
Seher, T. C. and Leptin, M. (2000). Tribbles, a cell-cycle brake that coordinates proliferation and morphogenesis during Drosophila gastrulation. Curr. Biol. 10,623 -629.[CrossRef][Medline]
Sigrist, S., Jacobs, H., Stratmann, R. and Lehner, C. F. (1995). Exit from mitosis is regulated by Drosophila fizzy and the sequential destruction of cyclins A, B and B3. EMBO J. 14,4827 -4838.[Abstract]
Spradling, A. (1993). Developmental genetics of oogenesis. In The Development of Drosophila melanogaster (ed. M. Bate), pp. 1-70. New York: Cold Spring Harbor Laboratory Press.
Tadros, W., Houston, S., Bashirullah, A., Cooperstock, R.,
Semotok, J., Reed, B. and Lipshitz., H. (2003).
Regulation of maternal transcript destabilization during egg activation in
Drosophila. Genetics
164,989
-1001.
Tan, L., Chang, J., Costa, A. and Schedl, P.
(2001). An autoregulatory feedback loop directs the localized
expression of the Drosophila CPEB protein Orb in the developing oocyte.
Development 128,1159
-1169.
Temme, C., Zaessinger, S., Meyer, S., Simonelig, M. and Wahle,
E. (2004). A complex containing the CCR4 and CAF1 proteins is
involved in mRNA deadenylation in Drosophila. EMBO J.
23,2862
-2871.
Tucker, M., Staples, R. R., Valencia-Sanchez, M. A., Muhlrad, D.
and Parker, R. (2002). Ccr4p is the catalytic subunit
of a Ccr4p/Pop2p/Notp mRNA deadenylase complex in Saccharomyces cerevisiae.
EMBO J. 21,1427
-1436.
Tucker, M., Valencia-Sanchez, M. A., Staples, R. R., Chen, J., Denis, C. L. and Parker, R. (2001). The transcription factor associated Ccr4 and Caf1 proteins are components of the major cytoplasmic mRNA deadenylase in Saccharomyces cerevisiae. Cell 104,377 -386.[Medline]
Van Buskirk, C., Hawkins, N. C. and Schupbach, T.
(2000). Encore is a member of a novel family of proteins and
affects multiple processes in Drosophila oogenesis.
Development 127,4753
-4762.
Verdone, L., Cesari, F., Denis, C. L., di Mauro, E. and Caserta,
M. (1997). Factors affecting Saccharomyces cerevisiae ADH2
chromatin remodeling and transcription. J. Biol. Chem.
272,30828
-30834.
Yue, L. and Spradling, A. C. (1992). hu-li tai shao, a gene required for ring canal formation during Drosophila oogenesis, encodes a homolog of adducin. Genes Dev. 6,2443 -2454.[Abstract]
Zaccai, M. and Lipshitz, H. D. (1996). Role of Adducin-like (hu-li tai shao) mRNA and protein localization in regulating cytoskeletal structure and function during Drosophila Oogenesis and early embryogenesis. Dev. Genet. 19,249 -257.[CrossRef][Medline]
|