1 Veterans Affairs Medical Center, Long Beach 90822; 2 Departments of Medicine and Physiology/ Biophysics, University of California School of Medicine, Irvine 92697; 3 Veterans Affairs Medical Center, Los Angeles 90073; and 4 Department of Medicine, University of California, Los Angeles, California 90024
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Previous studies have demonstrated the involvement of a specialized, Na+-dependent carrier-mediated system for biotin uptake in mammalian intestine. The molecular identity of the carrier protein, the Na+-dependent multivitamin transporter (SMVT), has recently been identified. Upon characterization of transcript expression in the rat intestine, four distinct transcript variants (I-IV) due to heterogeneity at the 5'-untranslated region were found (Chatterjee NS, Kumar CK, Ortiz A, Rubin SA, and Said HM. Am J Physiol Cell Physiol 277: C605-C613, 1999). This finding raised the possibility that multiple promoters may be involved in driving the transcription of the SMVT gene. To test this possibility, we cloned the 5' regulatory region of the SMVT gene by genome walking. A 6.5-kb genomic DNA fragment was identified and sequenced. Three putative promoters (P1, P2, and P3) that were separated by exons of the four previously identified transcript variants were, indeed, found. P1 was found to contain multiple putative regulatory regions like GATA-1, AP-1, AP-2, and C/EBP, including several repeats of purine-rich regions and two TATA-like elements. P2 and P3 were GC rich and also revealed the presence of many putative regulatory elements including several SP-1 consensus sequences. The functional identity of each promoter and the minimal regions required for its function were established by the luciferase assay following transfection of rat-derived cultured intestinal epithelial IEC-6 cells. The highest functional activity of the cloned promoters was found to be in the order of P1 > P2 > P3. These findings represent the first characterization of the 5' regulatory region of any mammalian SMVT gene and should assist in the understanding of transcriptional regulation of this important gene.
intestinal biotin transport; gene promoters; transport regulation; multiple variants
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
BIOTIN (VITAMIN H) IS AN ESSENTIAL micronutrient that acts as a coenzyme for four carboxylases that catalyze essential steps in four pathways involving fatty acid biosynthesis, gluconeogenesis, and catabolism of certain amino acids and fatty acids (2, 29). The importance of biotin for normal health and well being is underscored by the serious clinical abnormalities that result from its deficiency, which include neurological disorders, growth retardation, and dermal abnormalities (2, 6, 29, 34). In addition, evidence has been recently presented to suggest that marginal biotin deficiency could be teratogenic (34). Biotin deficiency and suboptimal levels are being reported with increased frequency in recent years. Deficiency of biotin occurs in conditions of inborn errors of biotin metabolism, following chronic use of certain anticonvulsant drugs, following long-term use of parenteral nutrition, during pregnancy, in a substantial number of alcoholics, and in patients with inflammatory bowel diseases (6, 10, 12, 29, 33, 34).
Humans and other mammals cannot synthesize biotin, and thus must obtain the vitamin from exogenous sources through absorption in the intestine. Therefore, the intestine plays a central role in determining and regulating normal biotin body homeostasis. Studies over the past 15 years from our laboratory and others have characterized different physiological and biochemical aspects of the intestinal biotin absorption process using a variety of intestinal preparations from different mammalian species (3, 18-27). These studies have shown the involvement of a Na+-dependent, concentrative, carrier-mediated system that is localized at the brush-border membrane of the intestinal absorptive cells. We have also shown that the intestinal biotin uptake process is regulated by dietary biotin deficiency levels (20) and by ontogeny (22). More recently, the intestinal biotin uptake system was also found to be utilized by the unrelated water-soluble vitamin pantothenic acid, and the metabolically important substrate lipoate (18, 23).
The molecular characteristics of the intestinal biotin absorption process of rat, human, and rabbit have been recently delineated by us and others (5, 16). Our studies in rat intestine have identified a cDNA clone that has an open reading frame identical to that of rat placenta, i.e., to that of sodium-dependent multivitamin transporter (SMVT) transcript (15). A significant heterogeneity, however, was found at the 5' untranslated region (5'-UTR) of the rat intestinal transcript with four distinct variants (I to IV) being identified. Furthermore, our findings have also shown the expression of the different variants to be tissue specific in nature (5). For example, variant II was found to be the predominant variant in the small and large intestine. However, this variant was not found in brain, lung, and stomach but was clearly expressed in kidney and liver (5). The finding of multiple variants for the SMVT gene suggests the possible involvement of multiple promoters that drive the expression of this gene. The latter suggestion is based on a substantial number of reports linking heterogeneity at the 5'-UTR of expressed transcripts with existence of multiple promoters (14, 17, 28). Our aim in this study was, therefore, to test this hypothesis and to confirm the existence of the multiple variants at the genomic level. The results obtained have indeed established the existence of multiple (three) promoters in the 5' regulatory region of the SMVT gene and showed that these promoters are separated by exons of the previously identified variants.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Materials. DNA restriction enzymes and Vent DNA polymerase were purchased from New England Biolabs (Beverly, MA). Routine biochemicals were all of molecular biology qualities and were purchased from Fisher Scientific (Tustin, CA). Cell culture reagents were purchased from Sigma Chemical (St. Louis, MO) and Life Technologies (Rockville, MD). Fetal bovine serum was obtained from Omega Scientific (Tarzana, CA). Kits used in this study were purchased from commercial sources as indicated.
Primers.
The primers used in different experiments are listed in Table
1. The primers were synthesized by Sigma
Genosys (The Woodlands, TX).
|
Genome walking. To obtain the genomic DNA sequence of SMVT that includes the promoter, upstream genome walking was performed using Rat GenomeWalker and Advantage Genomic PCR Kits (Clontech, Palo Alto, CA) according to the manufacturer's instructions. The first set of gene-specific primers (primers 1A and 1B, Table 1) was from the vicinity of the ATG initiation codon of the cloned intestinal cDNA. Primary PCR reactions were performed using the flanking adapter-specific primer 1 (AP1, supplied with the kit) and a flanking 3' gene-specific antisense primer, following the manufacturer's protocol and the supplied libraries. A 3.9-kb DNA fragment was obtained from the RDL-1 (EcoRV) library, amplified, and then diluted and used as template for nested secondary PCR using the nested adapter primer (AP2, supplied with the kit) and the respective nested gene-specific antisense primer (primer 1B). The fragment was cloned in pBluescript II SK+ for further characterization. A similar procedure was followed to move further in the upstream direction of the SMVT genomic DNA to obtain the entire 6.5-kb 5' upstream region. In cases where amplification of a genomic fragment proved initially unsuccessful, we suspected a GC-rich template and used a system containing both a reaction buffer and enzyme mix suitable for this purpose (Roche Biochemicals, Indianapolis, IN)
DNA sequence analysis. Subcloned DNA fragments obtained by genomic walking were sequenced with an ABI automated DNA sequencer by a commercial vendor (SeqWright, Houston, TX). Sequence alignment and searches were performed using the BLAST, the TRANSFAC, and the TFSEARCH databases (8, 32).
Primer extension analysis.
The transcription start sites of the rat SMVT gene were analyzed by
primer extension. Poly(A+) RNA isolated from rat small
intestine and colon were isolated as mentioned before (5).
Ten picomoles of primers 2R (from exon 1a, variant II), 3R (from exon
1b, variant IV), 4R (from exon 1c, variant I), and 5R (from exon 1d,
variant III) (Table 1) were end-labeled with [-32P]ATP
and T4 polynucleotide kinase (Promega). The primer extension reactions
were carried out with ~8 µg of the poly(A+) RNA, the
labeled primers, avian myeloblastosis virus reverse transcriptase, and
Primer Extension System (Promega) following manufacturer's procedure.
Primers 2R, 3R, and 5R were used with poly(A+) RNA from
small intestine, whereas primer 4R was used with poly(A+)
RNA from rat colon because this variant (i.e., variant I) was absent in
the small intestine (5). The extended product was resolved
on SequaGel-8 (National Diagnostics, Atlanta, GA) and visualized by
autoradiography. The products of primer extended were compared with an
X-174 HinfI DNA marker and 1-bp DNA sequence ladder (not shown).
Promoter-luciferase plasmid construct.
Three putative promoter regions (i.e., P1, P2, and P3) were amplified
by PCR using specific primers as indicated in Table 2. The PCR fragments were subsequently
subcloned upstream of the promoterless luciferase gene in the
pGL3-basic vector as described previously (4). The
constructs were confirmed by DNA sequencing.
|
Preparation of different deletion constructs of SMVT promoters.
Deletion constructs of the three SMVT promoter P1, P2, and P3 were
prepared as follows: the deletion constructs for P1 were prepared using
PCR (Table 2). The P1 deletion products were purified and subsequently
subcloned into the NheI and HindIII sites of pGL3-basic vector to generate the pGL3-P1 constructs. Deletion constructs for P2 and P3 were obtained by suitable restriction digestion to generate pGL3-P2
and pGL3-P3
constructs,
respectively. All constructs were verified with appropriate restriction
digestion and DNA sequencing.
Cell cultures and transfection. Promoter-luciferase reporter gene constructs were transfected into rat intestinal epithelial IEC-6 cells (ATCC, Manassas, VA). Approximately 2 × 106 cells were electroporated with 10 µg plasmid that contained the construct as described by us before (5). To normalize for the efficiency of transfection, these cells were also cotransfected with 300 ng of pRL-TK (Promega) plasmid along with the promoter constructs. Total cell lysate was prepared from cells 72 h following transfection, and firefly luciferase activity was measured in these cell extracts using a Turner Design 20/20 Luminometer (Sunnyvale, CA) and was normalized to Renilla luciferase activity in the same cell extract. Protein concentration was measured using Coomassie Plus-200 protein Assay Reagent (Pierce, Rockford, IL). Data presented here are means ± SE of at least three independent experiments and were expressed as relative expression over pGL3-basic, which was set arbitrarily at 1. Statistical analysis was performed using the Student's t-test.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Genomic organization of the 5' regulatory region of rat SMVT gene.
Using the genome walking technique described in the MATERIALS AND
METHODS, we obtained a 6.5-kb DNA fragment from the 5' regulatory region. The relative position of SMVT gene sequences is based on
numbering the "A" of the translation initiation codon sequence as
"1" with upstream sequences indicated as negative numbers. This
genomic fragment was sequenced and analyzed and was found to contain
exons of all the four recently identified transcript variants (i.e.,
variants I-IV) (Fig. 1). Because these
variants possess significant heterogeneity in the extreme 5' end of
their UTR, we have named the exons responsible for the heterogeneity as
exons 1a to 1d. The common 5'-UTR that exists in all variants was
marked as exon 2, the 60-bp placental exon was named exon 2a, and the
exon containing the ATG initiation codon was named as exon 3.
|
Determination of the transcription initiation site.
Primer extension analysis for transcription initiation site was carried
out using specific primers and poly(A)+ RNA from intestine
of rat as described in MATERIALS AND METHODS. The results
are shown in Fig. 2. Transcription
initiation sites for exon 1a were at 5078,
5133, and
5150; those
for exon 1b were at
4426,
4429, and
4442; those for exon 1c were
at
4218,
4220,
4225, and
4246; and those for exon 1d were at
4062,
4072,
4115, and
4126. No extended signal was observed
with reaction containing no RNA, which was used as a negative control (data not shown).
|
Promoter activity.
In an effort to establish the functionality of the three putative
promoters (P1, P2, and P3) of the biotin transporter SMVT gene, we
prepared three different constructs by cloning these putative promoter
regions into the promoterless pGL3-basic vector (Fig.
3) as described previously by Chatterjee
et al. (4). The pGL3-basic vector with or without the
putative promoter sequences as well as the vector expressing the
Renilla luciferase (pRL-TK) were cotransfected into rat
intestinal epithelial IEC-6 cells. Activity of the expressed firefly
luciferase from the putative promoter constructs and the basic vector
was normalized relative to the expressed Renilla luciferase
activity. The results showed that transfection of IEC-6 cells with
pGL3-P1, pGL3-P2, and pGL3-P3 expressed a 10.5-, 8.6-, and 5.2-fold
increase in luciferase activity, respectively, over the promoterless
pGL3-basic construct (P < 0.01 for all) (Fig. 3).
|
|
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Recent studies from our laboratory have identified four unique transcripts characterized as 5'-UTR variants expressed by the biotin transporter SMVT gene (5). These cDNA variants were found to be expressed in a tissue-specific manner (5). The existence of multiple 5'-UTR variants suggested the possible involvement of multiple promoters. To test this possibility, and to begin to understand the regulation of the intestinal biotin uptake process at the molecular level (the intestinal biotin absorption process is known to be regulated by dietary substrate levels and by ontogeny; Refs. 19 and 21), as well as the basis for the tissue-specific expression of the different transcript variants expressed by the SMVT gene, we cloned and characterized the 5' regulatory region of this gene. Our findings provided direct genomic sequence confirmation as the basis for the existence of the different variants, demonstrated their relative position in the upstream region of the biotin transporter SMVT gene, and provided information about their relation to regulatory regions of this gene.
We believe that the beginning of the 5' end of the regulatory region is
between 6110 and
6032, as sequence in this upstream region is found
to be shared by another rat gene (GenBank accession no.
M77850). The 3' end of the regulatory region of the SMVT gene
was marked by exon 1d and exon 2. Within the regulatory region, three
putative promoters (P1, P2, and P3) were identified that were separated
by exons of the four previously identified variants. P1 was found to be
972 bp in length, whereas P2 and P3 were 548 bp and 159 bp in length,
respectively. The identification of multiple promoters also suggests
the involvement of alternative splicing in the generation of the
previously identified distinct SMVT variants (5).
The putative P1 was found to lack any classic TATA or CAAT elements.
Instead, two TATA-like elements (4) were found at positions 5193 and
5224 of P1. In addition, P1 was also found to
have six putative GATA-1 binding sites. The existence of GATA-1 binding
sites in P1 of the SMVT gene is in line with our previous observations
of tissue-specific expression of this gene (5), as these
sites are known to be involved in tissue-specific expression of other
genes (8, 11, 12, 31). In addition to several putative
regulatory elements, P1 was found to contain several AP-1 elements and
a STAT-binding element, was purine rich, and contained 42%
G+C.
As to P2 and P3, these putative promoters were found to be GC rich,
containing 66% and 67% G+C, respectively. Again, no classic TATA or
CAAT elements were detected at the expected positions in either of
these promoters. P2 and P3, however, were found to contain several
SP-1, AP-2, and AP-3 elements, which may be involved in basal activity
of these promoters (see below). In addition, P2 and P3 were found to
contain an AP-1 element, whereas a nuclear factor-B (NF-
B)
element was found in P2. These putative regulatory elements
are known to be inducible under different cellular conditions, e.g.,
external stimuli (7), but their role in the regulation of
SMVT gene transcription is not known and needs further investigation.
The primer extension study suggests that the SMVT gene has multiple transcription initiation sites. From the sequence analysis, it was determined that the transcription initiation sites were located in close vicinity to the proximal TATA-like element in the case of P1 or from the proximal SP-1 elements in case of P2 and P3. The presence of several transcription initiation sites, which were preceded by multiple SP-1 binding sites and the lack of classic TATA or CAAT elements, is a characteristic of a constitutive promoter (1). The physiological significance of having multiple transcription start sites that are close to each other is unclear to us. Similar observations have been reported with other genes that utilize multiple promoters that lack classic TATA or CAAT elements and have multiple transcription start sites (30).
Functional activity of the putative promoter P1 was established by the promoterless luciferase reporter gene expression. Our results showed that among the three putative promoters, P1 was the most active promoter in cultured intestinal epithelial IEC-6 cells; it was followed by P2 and P3, respectively, as shown by the results displayed in Fig. 3. This finding is interesting when related to expression of the different variants in intestinal epithelial cells where variant II was found to be the predominant form (5). This is because the expression of this variant is driven by P1. It is not known at present, however, whether an enhancer is needed for proper functioning of P1 in the intestinal epithelial cells. This issue will be addressed in future investigation.
Our deletion studies to identify the minimum regions needed for basal
activity of the different promoters have shown that for P1, a 103-bp
sequence from 5311 to
5209 is essential for basal
transcription in IEC-6 cells. This region contains a putative TATA-like
element along with other regulatory elements including C/EBP, AP-1, and
GATA-1. This raises the possibility that these elements may be needed
for transcription of the SMVT gene; further studies, however, are
needed to test this possibility. The region from
6064 to
5763 was
also found to be important for P1 activity expressed in the intestinal
epithelial IEC-6 cells.
For P2 and P3, a 110-bp sequence from 4548 to
4439 and a
94-bp sequence from
4441 to
4319, respectively, were found to be needed for basal activity in the intestinal epithelial
IEC-6 cells. The minimal promoter region of P2 contains several SP-1 elements and other elements like AP-2, AP-4, Oct-1, and
NF-
B. The P3 minimal region also contains multiple SP-1 elements.
Again, it is not clear whether any of these putative regulatory
elements are involved in driving the basal transcription of the SMVT
gene in the intestine; further investigations are needed to address these issues. An interesting observation was the finding of a significant increase in P2 activity when a deletion was made from
4600 to
4548. This increase interrupted the progressive decrease in
the promoter activity found upon deletion from
4600 to
4548, and
suggests possible existence of a negative regulatory element in this
region. As stated earlier in this report, variant II is the most
predominantly expressed variant in small intestine. The observations
that P2 and P3 also show significant activity in IEC-6 cells may
indicate the possible existence of a silencer sequence somewhere in the gene.
In summary, results of this study represent the first characterization of the 5' regulatory region of a mammalian biotin transporter gene, SMVT. These findings should serve as a base for future investigation into the molecular regulation of the intestinal biotin absorption process and the pattern of tissue-specific expression of SMVT.
![]() |
ACKNOWLEDGEMENTS |
---|
The laboratory staff of H. M. Said and S. A. Rubin contributed equally to this work. We thank Scott Smith, Dr. Veedamali S. Subramanian, Alvaro Ortiz, and Ana-Paula E. Duarte for excellent technical help.
![]() |
FOOTNOTES |
---|
This study was supported by grants from the Department of Veterans Affairs and by National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-56061 and DK-58057.
Address for reprint requests and other correspondence: H. M. Said, VA Medical Center 151, Long Beach, CA 90822 (E-mail: hmsaid{at}uci.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 31 July 2000; accepted in final form 12 October 2000.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Azizkhan, JC,
Jensen DE,
Pierce AJ,
and
Wade M.
Transcription from TATA-less promoters: dihydrofolate reductase as a model.
Crit Rev Eukaryot Gene Expr
3:
229-254,
1993[Medline].
2.
Bonjour, JP.
Biotin.
In: Handbook of Vitamins: Nutritional Biochemical and Clinical Aspects, edited by Machlin LJ.. New York: Dekker, 1984, p. 403-435.
3.
Brown, B,
Selhub J,
and
Rosenberg IH.
Intestinal absorption of biotin in the rat.
J Nutr
116:
1266-1271,
1986[ISI][Medline].
4.
Chatterjee, N,
Zou C,
Osterman JC,
and
Gupta NK.
Cloning and characterization of the promoter region of a gene encoding a 67-kDa glycoprotein.
J Biol Chem
272:
12692-12698,
1997
5.
Chatterjee, NS,
Kumar CK,
Ortiz A,
Rubin SA,
and
Said HM.
Molecular mechanism of the intestinal biotin transport process.
Am J Physiol Cell Physiol
277:
C605-C613,
1999
6.
Dakshinamurti, K,
and
Chauhan J.
Regulation of biotin enzymes.
Annu Rev Nutr
8:
211-233,
1988[ISI][Medline].
7.
Faisst, S,
and
Meyer S.
Compilation of vertebrate-encoded transcription factors.
Nucleic Acids Res
20:
3-26,
1992[ISI][Medline].
8.
Gregory, RC,
Taxman DJ,
Seshasayee D,
Kensinger MH,
Bieker JJ,
and
Wojchowski DM.
Functional interaction of GATA1 with erythroid Kruppel-like factor and Sp1 at defined erythroid promoters.
Blood
87:
1793-1801,
1996
9.
Heinemeyer, T,
Wingender E,
Reuter I,
Hermjakob H,
Kel AE,
Kel OV,
Ignatieva EV,
Ananko EA,
Podkolodnaya OA,
Kolpakov FA,
Podkolodny NL,
and
Kolchanov NA.
Databases on transcriptional regulation: TRANSFAC, TRRD and COMPEL.
Nucleic Acids Res
26:
362-367,
1998
10.
Krause, KH,
Berlit P,
and
Bonjour JP.
Impaired biotin status in anticonvulsant therapy.
Ann Neurol
12:
485-486,
1982[ISI][Medline].
11.
Lemarchandel, V,
Ghysdael J,
Mignotte V,
Rahuel C,
and
Romeo PH.
GATA and Ets cis-acting sequences mediate megakaryocyte-specific expression.
Mol Cell Biol
13:
668-676,
1993[Abstract].
12.
Migliaccio, AR,
Migliaccio G,
Ashihara E,
Moroni E,
Giglioni B,
and
Ottolenghi S.
Erythroid-specific activation of the distal (testis) promoter of GATA1 during differentiation of purified normal murine hematopoietic stem cells.
Acta Haematol
95:
229-235,
1996[ISI][Medline].
13.
Mock, DM,
deLorimer AA,
Liebman WM,
Sweetman L,
and
Baker H.
Biotin deficiency: an unusual complication of parenteral alimentation.
N Engl J Med
304:
820-823,
1981[ISI][Medline].
14.
Nakamuta, M,
Oka K,
Krushkal J,
Kobayashi K,
Yamamoto M,
Li WH,
and
Chan L.
Alternative mRNA splicing and differential promoter utilization determine tissue-specific expression of the apolipoprotein B mRNA editing protein (Apobec) gene in mice. Structure and evaluation of Apobec and related nucleotide deaminases.
J Biol Chem
270:
13042-13056,
1995
15.
Prasad, PD,
Wang H,
Kekuda R,
Fujita T,
Feis YJ,
Devoe LD,
Leibach FH,
and
Ganapathy V.
Cloning and functional expression of a cDNA encoding a mammalian-sodium dependent vitamin transporter mediating the uptake of pantothenate, biotin and lipoate.
J Biol Chem
273:
7501-7506,
1998
16.
Prasad, PD,
Wang H,
Huang W,
Fei YJ,
Leibach FH,
Devoe LD,
and
Ganapathy V.
Molecular and functional characterization of the intestinal Na+-dependent multivitamin transporter.
Arch Biochem Biophys
366:
95-106,
1999[ISI][Medline].
17.
Rajagopalan, S,
Wan DF,
Habib GM,
Sepulveda AR,
McLead MR,
Lebovitz RM,
and
Leiberman MW.
Six mRNA with different 5' ends are encoded by a single gamma-glutamyltransferase gene in the mouse.
Proc Natl Acad Sci USA
90:
6179-6183,
1993[Abstract].
18.
Said, HM.
Cellular uptake of biotin: mechanisms and regulation.
J Nutr
129:
490S-493S,
1999[ISI][Medline].
19.
Said, HM,
and
Derweesh I.
A carrier-mediated mechanism for biotin transport in rabbit intestine: studies with brush border membrane vesicles.
Am J Physiol Regulatory Integrative Comp Physiol
261:
R94-R97,
1991
20.
Said, HM,
Mock DM,
and
Collins JC.
Regulation of intestinal biotin transport in the rat: effect of biotin deficiency and supplementation.
Am J Physiol Gastrointest Liver Physiol
256:
G306-G311,
1989
21.
Said, HM,
and
Mohammedkhani R.
Involvement of histidine residues and sulfhydryl groups in the function of the biotin transport carrier of rabbit intestinal brush-border membrane.
Biochim Biophys Acta
1107:
238-244,
1992[ISI][Medline].
22.
Said, HM,
and
Redha R.
Ontogenesis of the intestinal transport of biotin in the rat.
Gastroenterology
94:
68-72,
1988[ISI][Medline].
23.
Said, HM,
Ortiz A,
McCloud E,
Dyer D,
Moyer MP,
and
Rubin S.
Biotin uptake by human colonic epithelial NCM460 cells: a carrier-mediated process shared with pantothenic acid.
Am J Physiol Cell Physiol
275:
C1365-C1371,
1998
24.
Said, HM,
and
Redha R.
Biotin transport in brush border membrane vesicles of rat small intestine.
Biochim Biophys Acta
945:
195-201,
1988[ISI][Medline].
25.
Said, HM,
Redha R,
and
Nylander W.
A carrier-mediated, Na+ gradient-dependent transport system for biotin in human intestinal brush border membrane vesicles.
Am J Physiol Gastrointest Liver Physiol
253:
G631-G636,
1987
26.
Said, HM,
Redha R,
and
Nylander W.
Biotin transport in basolateral membrane vesicles of human intestine.
Gastroenterology
94:
1157-1163,
1988[ISI][Medline].
27.
Said, HM,
Redha R,
and
Nylander W.
Biotin transport in human intestine: site of maximum transport and effect of pH.
Gastroenterology
95:
1312-1317,
1988[ISI][Medline].
28.
Schibler, U,
and
Sierra F.
Alternative promoters in developmental gene expression.
Annu Rev Genet
21:
237-257,
1987[ISI][Medline].
29.
Sweetman, L,
and
Nyhan WL.
Inheritable biotin-treatable disorders and associated phenomena.
Annu Rev Nutr
6:
314-343,
1986.
30.
Tolner, B,
Roy K,
and
Sirotnak FM.
Structural analysis of the human RFC-1 gene encoding a folate transporter reveals multiple promoters and alternatively spliced transcripts with 5' end heterogeneity.
Gene
211:
331-341,
1998[ISI][Medline].
31.
Tournamille, C,
Colin Y,
Cartron JP,
and
Le Van Kim C.
Disruption of a GATA motif in the Duffy gene promoter abolishes erythroid gene expression in Duffy-negative individuals.
Nat Genet
10:
224-228,
1995[ISI][Medline].
32.
Wingender, E,
Chen X,
Hehl R,
Karas H,
Liebich I,
Matys V,
Meinhardt T,
Pruss M,
Reuter I,
and
Schacherer F.
TRANSFAC: an integrated system for gene expression regulation.
Nucleic Acids Res
28:
316-319,
2000
33.
Wolf, B,
Heard GS,
Jefferson LG,
Proud VK,
Nance WI,
and
Weissbecker KA.
Clinical findings in four children with biotinidase deficiency detected through a state-wide neonatal screening program.
N Engl J Med
313:
16-19,
1985[Abstract].
34.
Zempleni, J,
and
Mock DM.
Marginal biotin deficiency is teratogenic.
Proc Soc Exp Biol Med
223:
14-21,
2000