Department of Genetics, University of Cambridge, Cambridge, England
Abstract
It has recently been discovered that the Alcohol dehydrogenase and Alcohol dehydrogenaserelated genes of Drosophila melanogaster and closely related species constitute a single transcription unit and that the Alcohol dehydrogenaserelated gene is exclusively expressed from a dicistronic mRNA. Here, we show that in Drosophila lebanonensis, subgenus Scaptodrosophila, Adhr is also transcribed as a dicistronic transcript with Adh. Using degenerate primers designed on the sequence of the known ADHR proteins, we have been able to amplify and sequence a partial sequence of Adhr in species representative of the whole subgenus Drosophila. This has allowed the study of the organization and expression of Adhr in Drosophila buzzatii. We find that in D. buzzatii Adhr is transcribed as a monocistronic transcript. Adh and Adhr are believed to originate by duplication, and our data suggest that the cotranscription of these two genes was the primitive state, and that their independent transcription in the subgenus Drosophila is derived. We can rationalize the D. buzzatii condition as being correlated with the two genes evolving independent transcriptional control. However, why these two genes with clear divergence in the functions of their proteins should remain cotranscribed in groups as divergent as the subgenus Sophophora and the subgenus Scaptodrosophila remains a mystery.
Introduction
Polycistronic transcription, that is, the coexpression of more than one open reading frame from a single promoter to make a polycistronic mRNA, is uncommon in eukaryotes, and its significance is not very well understood (Blumenthal 1998). Known examples seem to fall into three classes. In one, the clustered genes are very different in sequence and the significance of polycistronic expression is that the products function in the same pathway. This is so in the case of the cyclophilin and the disulphide isomerase genes in Caenorhabditis elegans. These genes are cotranscribed, and both are needed to modify the initial translation product of the collagen gene (Page 1997
). The lin-15B and lin-15A genes play a role in vulva development in C. elegans; they are unrelated in sequence and cotranscribed (Clark, Lu, and Horvitz 1994
; Huang, Tzou, and Sternberg 1994
). An example in Drosophila melanogaster is that of the two genes cotranscribed at the stoned locus (Andrews et al. 1996
). In the second type of example, two unrelated proteins are also produced, but there is no clear reason for their coexpression. In C. elegans, 25% of gene expression is achieved by means of polycistronic transcription, but in only few of these instances has this been shown to have a functional meaning (Spieth et al. 1993
; Zorio et al. 1994
; Blumenthal 1995
; Blumenthal and Spieth 1996
). In mice, the growth/differentiation factor-1 (GDF-1) is cotranscribed downstream of the gene UOG-1, a membrane protein not known to be related in function (Lee 1991
). In the third type of example, there is polycistronic transcription of two genes that are clearly related in sequence and have, we presume, evolved by gene duplication. Two cases in D. melanogaster are those of Alcohol dehydrogenase (Adh) and Alcohol dehydrogenase related (Adhr) (Brogna and Ashburner 1997
) and sesB and Ant2 (Zhang et al. 1999
). In both cases, the gene pairs are clearly related in sequence, although in the case of Adh and Adhr, they are apparently not related in function.
The evidence that the Adh and Adhr genes of D. melanogaster evolved by duplication is twofold: first, there is their sequence similarity (about 40% amino acid identity); second, they have a common intron/exon structure (Rat, Veuille, and Lepesant 1991
; Schaefer and Aquadro 1987
). This duplication is clearly very old, perhaps as old as 180 Myr (Russo, Takezaki, and Nei 1995
). The function of Adh is well known: it is the detoxification of dietary alcohols (Chambers 1988
; Ashburner 1998
). Adhr is of unknown function, although it seems not to be a NAD-dependent dehydrogenase (Jeffs, Holmes, and Ashburner 1994
). Within the genus Drosophila, the ADHR protein is evolving slightly more slowly than the ADH protein (Albalat, Marfany, and González-Duarte 1994
).
The dicistronic transcription of Adh and Adhr has been studied in great detail in D. melanogaster (Brogna and Ashburner 1997
). In this species, there are only two different kinds of transcript: one (
1 kb) includes only Adh, and the other (
2 kb) is dicistronic, including both genes. Adh and Adhr in D. melanogaster are arranged in tandem, being separated by only
300 bp. The dicistronic transcript includes this intergenic region. The dicistronic transcript is rare, with its abundance being only about 1% that of the monocistronic Adh transcript. The relative level of the two transcripts is controlled by the efficiency of transcriptional read-through of the termination and polyadenylation signals in the intergenic region (Brogna and Ashburner 1997
).
Quite a lot is known about the distribution of the Adhr gene in the genus Drosophila (Russo, Takezaki, and Nei 1995
; Amador and Juan 1999
); it has been described in the Scaptodrosophila subgenus and in many species of the Sophophora subgenus, but in two only species of the Drosophila subgenus (in D. immigrans and, recently, in D. funebris). One striking fact is that although the duplication predates the radiation of the genus Drosophila (4065 Myr; Powell 1997
, pp. 282285), Adhr has not been found in many species of Drosophila, despite considerable study of their Adh genes (Oakeshott et al. 1982
; Sullivan, Atkinson, and Starmer 1990
; Nurminsky et al. 1996
; Begun 1997
). Here, using degenerate primers designed on the sequence of the known ADHR proteins, we were able to amplify a partial sequence of Adhr in species representative of the whole subgenus Drosophila. This allowed the study of the organization and expression of Adhr in D. buzzatii, of the subgenus Drosophila. The Adh region in D. buzzatii is very different from that in D. melanogaster, since the Adh gene itself has been duplicated (Oakeshott et al. 1982
; Sullivan, Atkinson, and Starmer 1990
; Begun 1997
). We show that in this species, Adh and Adhr are not transcribed as a dicistronic message. We also studied the expression of Adhr in Drosophila lebanonensis (subgenus Scaptodrosophila), thought to be an early radiation in the subfamily Drosophilinae (Tatarenkov et al. 1999
). In this species, Adh and Adhr are transcribed as a dicistronic mRNA, as in D. melanogaster. These results suggest that the cotranscription of Adh and Adhr is the primitive state and that their independent transcription in the subgenus Drosophila is derived.
Materials and Methods
Drosophila Stocks and Isolation of DNA
Genomic DNA of adult flies was isolated from D. mulleri, D. hydei, D. repleta, and D. buzzatii (repleta species group); D. virilis and D. montana (virilis species group); D. robusta (robusta species group); D. funebris (funebris species group); and the Hawaiian species D. biseriata and D. gymnobasis. Stocks of all of these species were obtained from Bowling Green Centre, except for D. buzzatii (2st 39.13; Betrán 1996
). The Puregene kit protocol was used for the isolation of genomic DNA.
PCR Protocols
Degenerate primers based on the amino acid sequences of the known ADHR proteins were designed to partially amplify the Adhr gene in different species. The primers were designed from regions that were conserved among ADHR proteins but not between ADHR and ADH. Polymerase chain reaction (PCR) amplifications were done at an annealing temperature of 45°C. The initial PCR product (500700 bp) was isolated from the gel, cloned in a T-end easy vector (Promega), and sequenced. Further primers were designed based on this sequence and used to obtain two further PCR amplifications (except for D. buzzatii, for which genomic sequence was obtained; see below). The primers are given in table 1
.
|
Sequencing Procedures
DNA sequencing was done on an ABI automated DNA sequencer (Applied Biosystems) with fluorescent DyeDeoxy terminator reagents. PCR products were sequenced either directly after gel purification (Qiagen kit) or after cloning in a T-end easy vector. Phage lambda cloned DNA was sequenced using PCR primers.
Sequence Alignment and Phylogenetic Inference
The coding regions of the sequences were aligned using CLUSTAL W (Thompson, Higgins, and Gibson 1994
) and revised by eye. MEGA, version 1.01 (Kumar, Tamura, and Nei 1994
), was used to manipulate the files. The DNAML program of PHYLIP, version 3.5c (Felsenstein 1993
), was used to build maximum-likelihood (ML) trees using information from all positions. PUZZLE, version 4.0 (Strimmer and von Haeseler 1996
), was used to estimate from the data the transition/transversion parameter used in the DNAML program. TREEVIEW (Page 1996
) was used to display the phylogenies. The sequence of the Fat body protein 2 (Fbp2) protein of D. melanogaster was used as an outgroup in all the comparisons. This gene is clearly a member of the Adh/Adhr gene family, sharing both sequence similarities and conserved intron positions (Rat, Veuille, and Lepesant 1991
). In sequence, it is roughly equally distant from both ADH and ADHR.
The DDBJ/EMBL/GenBank accession numbers of sequences used in the analysis are as follows: Adh and Adhr of D. melanogaster, X78384; Fbp2 of D. melanogaster, S57693; Adh and Adhr of D. lebanonensis, M97637; Adh1 and Adh2 of D. hydei, X58694; AdhF of D. hydei, U76469; Adh1 and Adh2 of D. buzzatii, U65746; AdhF of D. buzzatii, U77607; Adh1 and Adh2 of D. mulleri, X03048; AdhF of D. mulleri, U76478; Adh1 and Adh2 of D. virilis, U26846; Adh1 of D. montana, U26842; and Adh2 of D. montana, U26845.
RNA Extraction
Total RNA preparations were made as described in Ashburner (1989)
, except that tissues were homogenized directly, without previous grinding in liquid nitrogen.
RT-PCR Experiments in D. lebanonensis
Total RNA was prepared from D. lebanonensis larvae and adults. Single-strand complementary DNA (cDNA) was synthesized using Superscript (Gibco-BRL). Primer L6 was used to synthesize the D. lebanonensis Adhr cDNA. Adhr cDNA was amplified and sequenced using primers L1 and L2 in the Adh region and L4 and L5 in the Adhr region. Primers are described in figure 1A.
|
The full-length sequence of the D. buzzatii Adhr transcript in larvae was obtained by sequencing the products of 5' and 3' rapid amplification of cDNA ends (RACE) experiments. Single-strand cDNA was synthesized from total RNA using Superscript (Gibco-BRL). Oligo-(dT) was used to prime the synthesis of the 3' end of the cDNA. Oligo-(dT) and the specific primers R1 and R4 were used to PCR-amplify the 3' end. The nested PCR product was subcloned and sequenced. Primer R7 was used to synthesize the 5' end of the cDNA. The cDNA was tailed with dCTP by using terminal transferase (Boeringher). Oligo-(dG) and the nested primers R5 and R3 were used to PCR-amplify the 5' end of the cDNA. Primers are described in figure 1B.
As an alternative method to determine the ends of the D. buzzatii Adhr transcript, we used circular retrotranscription PCR (cRT-PCR; Brogna 1999
). mRNA was decapped using tobacco acid pyrophosphatase (TAP). The decapped RNA was then ligated using T4 RNA ligase. To remove the poly(A), the decapped mRNA was hybridized with oligo-T (20°C for 20 min) and the hybrid duplex was digested with RNase H. Primer R4 (see fig. 1
) was used to synthesize a cDNA for Adhr across the poly(A) junction. Primers R3 and R6 (fig. 1
) were used to amplify the Adhr cDNA. A nested amplification was achieved by using primers R2 and R8. Products were sequenced.
mRNA In Situ Hybridization
The temporal patterns of expression of Adh and Adhr were studied in D. buzzatii and D. lebanonensis embryos, larvae, and adult tissues. Embryos were collected in cages, dechorionated using 50% bleach, and fixed in 4% formaldehyde. Hybridization was conducted as described in Tautz and Pfeifle (1989)
after removal of the vitelline membrane using methanol. Larval and adult tissues were dissected in PBS and fixed in ice-cold 4% formaldehyde in phosphate buffer. Hybridization was performed as described in Cubas et al. (1991)
.
Probes were DIG-dUTPlabeled by random priming. Increasing the amount of the random primer allowed the production of very small probes that easily entered cells. The probes used in D. buzzatii were the subcloned PCR products made using primers A1 and A3 for Adh and primersR1 and R7 for Adhr. The probes used for D. lebanonensis were also amplified by PCR from clones of the Adh region (kindly provided by Elvira Juan). Primers L2 and L3 were used to amplify the Adh probe, and primers D1 and D4 were used to amplify the Adhr probe (see fig. 1A and table 1 ).
Results
Adhr in the Subgenus Drosophila
Using degenerate primers, we partially sequenced Adhr from 10 species of the subgenus Drosophila. An ML tree of these sequences (fig. 2
) shows them to cluster apart from the Adh sequences, with which they show only 40%45% nucleic acid sequence identity.
|
We obtained the complete sequence of the Adhr gene in D. buzzatii (fig. 3A ). In structure, this gene is very similar to Adhr of D. melanogaster, although the buzzatii gene has a rare splice donor sequence (GC) in intron 2. The predicted protein sequence of this gene is 82% identical to those of D. melanogaster and D. lebanonensis.
|
Adhr Is Monocistronic in D. buzzatii
Figure 4A
shows a Northern blot using total RNA from larvae and adults; it shows transcripts for both Adh and Adhr in both larvae and adults. The size of these transcripts suggests that both are monocistronic. There is no cross-hybridization between Adh and Adhr (fig. 4B
). As in D. melanogaster (Brogna and Ashburner 1997
), the Adh transcript is much more abundant than that of Adhr. The monocistronic structure of the Adhr mRNA was confirmed by a full-length cDNA sequence (fig. 3A
) and independent sequencing of its 5' and 3' ends by cRT-PCR.
|
Temporal Pattern of Expression
The tissues that show ADH activity are the larval and adult fat body and gut, the larval Malpighian tubules, and the adult ovaries (Shafer 1992
). We studied the distribution of Adh and Adhr transcripts in D. buzzatii embryos, larvae, and adult tissues by in situ hybridization (data not shown). Adh and Adhr transcripts were detected in all of the tissues where ADH activity has been detected. However, the location of the two transcripts in embryos differs. Adhr is expressed only in the gastric caecae, while Adh shows a residual maternal expression and is expressed early in the yolk and mesoderm and later in the fat body.
Adhr in D. lebanonensis
cDNA Analysis
We carried out an RT-PCR to directly check the existence of a dicistronic transcript of Adh and Adhr with total larval and adult RNA of D. lebanonensis. A PCR product from larvae and adults of the size expected for a dicistronic transcript was cloned and sequenced. The sequence confirmed that this was a dicistronic Adh-Adhr transcript, including the intergenic region; the intron sequences were spliced. Since a monocistronic transcript for Adh has been detected in D. lebanonensis (Albalat and González-Duarte 1993
; Juan, Papaceit, and Quintana 1994
), we believe that, as in D. melanogaster, there are two transcripts: one monocistronic transcript that encodes ADH and one dicistronic transcript that encodes both ADH and ADHR.
Temporal Pattern of Expression
ADH activity and/or transcription has previously been detected in larvae, pupae, and adults of D. lebanonensis but not in embryos (Juan, Papaceit, and Quintana 1994
). We detected Adh and Adhr transcripts in D. lebanonensis in embryos, larvae, and adult tissues (data not shown). The localization of the transcripts in larvae and adults was similar to that observed previously. In embryos, both genes are expressed in a striking segmental pattern from stage 13 onward.
Discussion
In D. melanogaster, the duplicate genes Adh and Adhr are transcribed as a dicistronic mRNA, with the ADHR protein being translated, most likely, after internal ribosome entry in the 300-bp region between their coding sequences (Brogna and Ashburner 1997
). We find that Adhr is widespread, probably universal, in the subfamily Drosophilinae. In the subgenus Drosophila, as represented by D. buzzatii, it may be close to Adh but transcribed independently of that gene. In the subgenus Scaptodrosophila, these two genes are very closely linked (just 287 bp apart; Juan, Papaceit, and Quintana 1994
), and, as in D. melanogaster Adhr and closely related species, they are transcribed as a dicistronic mRNA. Figure 5
summarizes what is currently known about the distribution and transcription of the Adhr gene in the genus Drosophila.
|
There is some evidence that the Adh genes of drosophilids have evolved from a gene encoding a member of the short-chain dehydrogenase family with a different function (Ashburner 1998
; S. Brogna, personal communication). This gene may have been Adhr, or it may have been one of the several other genes encoding members of this protein family in drosophilids. The phylogeny supports the view that the ancient organization after Adh/Adhr duplication consisted of two very close genes. Our data suggest that the cotranscription of Adh and Adhr is the primitive state and that their independent transcription in the subgenus Drosophila is derived. While we can rationalize the latter condition as being correlated with the two genes evolving independent transcriptional control, why they should remain cotranscribed in groups as divergent as the subgenus Sophophora and the subgenus Scaptodrosophila remains a mystery, a mystery compounded by the clear divergence in function of their proteins.
Given that these two genes are believed to originate from an adjacent duplication, most parsimoniously, the two genes have been cotranscribed since the duplication event and have evolved different functions despite this.
Supplementary Material
Partial or complete sequences of Adhr in the 10 species of the subgenus Drosophila have been deposited in GenBank with the accession numbers AF260690AF260699. Alignments have been submitted to the EMBL sequence alignment database (ds42702).
Acknowledgements
We are indebted to the Saverio Brogna for his generosity. His suggestions and support during the experimental work were of great help. Sam Loh, Stefan Oehler, and Natalia Sánchez-Soriano were also very helpful at different stages in the work. Mario Cáceres and Alfredo Ruiz kindly supplied the genomic phage library of D. buzzatii. Elvira Juan kindly provided clones for Adh and Adhr of D. lebanonensis. We thank Antonio Barbadilla, Saverio Brogna, Martin Kreitman, Manyuan Long, Alfredo Ruiz, two anonymous reviewers, and Wolfgang Stephan for critically reading the manuscript. E.B. was supported by a postdoctoral Marie Curie Research and Mobility fellowship from European Commission Work was supported by an MRC Programme grant to M.A., Steve Russell, and David Gubb.
Footnotes
Wolfgang Stephan, Reviewing Editor
1 Present address: Department of Ecology and Evolution, University of Chicago.
1 Abbreviations: Adh, Alcohol dehydrogenase; Adhr, Alcohol dehydrogenase related; cDNA, complementary DNA to an RNA obtained by retrotranscription; PCR, polymerase chain reaction; RT-PCR, PCR in a cDNA.
2 Keywords: duplication
dicistronic transcription
Drosophila
Adh,
Adhr.
3 Address for correspondence and reprints: Esther Betrán, Department of Ecology and Evolution, University of Chicago, 1101 East 57th Street, Chicago, Illinois 60637. E-mail: ebetran{at}midway.uchicago.edu
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