Xenopus laevis Contains Two Nonallelic Preproinsulin Genes

Novel Human Insulin Isoforms and Cα-Peptide Product in Islets of Langerhans and Choroid Plexus

Human insulin (INS) gene diverged from the ancestral genes of invertebrate and mammalian species millions of years ago. We previously found that mouse insulin gene (Ins2) isoforms are expressed in brain choroid plexus (ChP) epithelium cells, where insulin secretion is regulated by serotonin and not by glucose. We further compared human INS isoform expression in postmortem ChP and islets of Langerhans. We uncovered novel INS upstream open reading frame isoforms and their protein products. In addition, we found a novel alternatively spliced isoform that translates to a 74-amino acid (AA) proinsulin containing a shorter 19-AA C-peptide sequence, herein designated Cα-peptide. The middle portion of the conventional C-peptide contains β-sheet (GQVEL) and hairpin (GGGPG) motifs that are not present in Cα-peptide. Islet amyloid polypeptide (IAPP) is not expressed in ChP, and its amyloid formation was inhibited in vitro more efficiently by Cα-peptide than by C-peptide. Of clinical relevance, the ratio of the 74-AA proinsulin to proconvertase-processed Cα-peptide was significantly increased in islets from type 2 diabetes mellitus autopsy donors. Intriguingly, 100 years after the discovery of insulin, we found that INS isoforms are present in ChP from insulin-deficient autopsy donors.

Evolution of the Insulin Gene: Changes in Gene Number, Sequence, and Processing

Insulin has not only made major contributions to the field of clinical medicine but has also played central roles in the advancement of fundamental molecular biology, including evolution. Insulin is essential for the health of vertebrate species, yet its function has been modified in species-specific manners. With the advent of genome sequencing, large numbers of insulin coding sequences have been identified in genomes of diverse vertebrates and have revealed unexpected changes in the numbers of genes within genomes and in their sequence that likely impact biological function. The presence of multiple insulin genes within a genome potentially allows specialization of an insulin gene. Discovery of changes in proteolytic processing suggests that the typical two-chain hormone structure is not necessary for all of inulin's biological activities.

Retinoic acid-induced expression of Hnf1b and Fzd4 is required for pancreas development in Xenopus laevis

Retinoic acid (RA) is required for pancreas specification in Xenopus and other vertebrates. However, the gene network that is directly induced by RA signalling in this context remains to be defined. By RNA sequencing of in vitro-generated pancreatic explants, we identified the genes encoding the transcription factor Hnf1β and the Wnt-receptor Fzd4/Fzd4s as direct RA target genes. Functional analyses of Hnf1b and Fzd4/Fzd4s in programmed pancreatic explants and whole embryos revealed their requirement for pancreatic progenitor formation and differentiation. Thus, Hnf1β and Fzd4/Fzd4s appear to be involved in pre-patterning events of the embryonic endoderm that allow pancreas formation in Xenopus.

Comparative analysis of insulin gene promoters: implications for diabetes research

DNA sequences that regulate expression of the insulin gene are located within a region spanning approximately 400 bp that flank the transcription start site. This region, the insulin promoter, contains a number of cis-acting elements that bind transcription factors, some of which are expressed only in the beta-cell and a few other endocrine or neural cell types, while others have a widespread tissue distribution. The sequencing of the genome of a number of species has allowed us to examine the manner in which the insulin promoter has evolved over a 450 million-year period. The major findings are that the A-box sites that bind PDX-1 are among the most highly conserved regulatory sequences, and that the conservation of the C1, E1, and CRE sequences emphasize the importance of MafA, E47/beta2, and cAMP-associated regulation. The review also reveals that of all the insulin gene promoters studied, the rodent insulin promoters are considerably dissimilar to the human, leading to the conclusion that extreme care should be taken when extrapolating rodent-based data on the insulin gene to humans.

Early developmental expression of two insulins in zebrafish (Danio rerio)

We have cloned a second insulin gene in zebrafish and studied temporal and spatial expression of two zebrafish insulin genes. Zebrafish insulin-a (insa) and -b (insb) mRNAs are derived from two different DNA contigs on chromosomes 5 and 14, respectively, representing two different insulin genes. Real-time PCR studies suggest that insa is a maternal and also a postzygotic transcript. insa was observed at 1 h postfertilization (hpf) and was rapidly degraded by 6 and 12 hpf but induced at 24 hpf (i.e., after pancreas formation). Expression levels at 24 hpf were approximately 220-fold higher than at 6 hpf and were significantly different from earlier time points. At 72 hpf (at time of hatching), zebrafish insa mRNA levels tended to be higher than at 24 hpf and were approximately 727-fold higher compared with 6 hpf. This further increase in insa expression may be one of the many rapid physiological changes associated with hatching. insb expression was observed from 1 hpf and was significantly decreased from 12 hpf onward. Its expression levels at 12 and 24 hpf were approximately twofold and sixfold lower, respectively, compared with expression at 6 hpf. insb expression levels at 48 hpf were significantly lower than at 24 hpf but not different from 72 hpf. Expression levels at 72 hpf were approximately 61-fold lower than at 6 hpf. In situ hybridization studies showed insb expression in proliferating blastomeres at 3 and 4 hpf. At later time points, insb expression was restricted to the brain and pancreas (24 and 48 hpf). insa expression was observed in the pancreas at 24 and 48 hpf. Expression of insb in blastomeres and head suggests that insb could be acting as a pro-growth, survival, and neurotrophic factor during development. Pancreatic insa and insb may both be involved in regulation of glucose homeostasis as in mammals.

Expression of the insulin receptor-related receptor is induced by the preovulatory surge of luteinizing hormone in thecal-interstitial cells of the rat ovary

The insulin receptor-related receptor (IRR) is a member of the insulin receptor family that, on its own, recognizes neither insulin nor any of the identified insulin-related peptides. In both the nervous system and peripheral tissues, IRR mRNA is detected in cells that also express trkA, the nerve growth factor tyrosine kinase receptor. In the ovary, the trkA gene is transiently activated in thecal-interstitial cells of large antral follicles at the time of the preovulatory surge of gonadotropins. The present study shows that the IRR gene is expressed in the same ovarian compartment, that IRR mRNA content increases strikingly in these cells in the afternoon of the first proestrus, and that--as in the case of trkA mRNA--the increase is caused by gonadotropins. The IRR mRNA species primarily affected is that encoding the full-length receptor; its increased abundance was accompanied by a corresponding change in IRR protein content. An extensive molecular search using several approaches, including the screening of cDNA libraries and PCR amplification with degenerate primers, did not yield an IRR ligand. Phylogenetic analysis of 20 insulin-related sequences and 15 relaxin family peptides from selected vertebrates indicated that the mammalian genome is unlikely to contain an additional ligand expressed from a distinct gene that is closely related to the insulin family. Although the functional nature of the relationship between IRR and trkA receptors is unknown, the remarkable temporal and spatial specificities of their coordinated expression in the ovary before ovulation suggests that they target a functionally related set of downstream events associated with the ovulatory process.

Identification of multiple cytochrome P450 genes belonging to the CYP4 family in Xenopus laevis: cDNA cloning of CYP4F42 and CYP4V4

In order to obtain cDNA clones coding for CYP4 proteins in frog Xenopus laevis, degenerate primers were designed utilizing the conserved sequences of known CYP4s and were used to amplify partial cDNA fragments from liver mRNA. Five new CYP genes were identified. Three of these genes, XL-1, -2 and -3, were assigned to the CYP4T subfamily found previously in fish and amphibians. The other two genes, XL-4 and XL-5, were quite similar to CYP4F and CYP4V subfamilies, respectively. Subsequently, two full-length cDNA clones corresponding to XL-4 and XL-5 were isolated and characterized. The resultant cDNAs, designated as CYP4F42 and CYP4V4, had open reading frames encoding proteins of 528 and 520 residues, respectively. RT-PCR analysis indicated that the expression of CYP4F42 was limited to the liver, kidney, intestine and brain. In contrast, CYP4V4 mRNA was expressed ubiquitously.

Evolution of preproinsulin gene in birds

The coding region of the preproinsulin gene has been cloned and partly sequenced in a variety of marine and terrestrial birds (28 species). All genes showed the "ancestral" structure with a large intron-2. The size of intron-2 changed considerably during the evolution of birds (2.4-4.2kb). The hydrophobicity of signal peptides was conserved. Bird C-peptides were predicted to be 28 amino acids long, but circulating C-peptides would be only 26 amino acids long, with Passer as a possible exception. Bird C-peptides were found to lack the sequences identified in mammals as responsible for peptide bioactivity and the structure of the central part. In contrast, predicted insulin sequences were highly conserved. Only two types of analog were identified: the hypoactive form (GluA8), present only in Anseriformes and the hyperactive form (His A8), present in all other species. Based on 3'-nucleotide sequence analysis (extending into intron-2), birds appeared to be monophyletic. Five groups were clearly identified: Paleognathae, Galliformes, Anseriformes, Passeriformes, and Charadriiformes. Paleognathae were suggested as the basal group, supporting the traditional view of avian evolution. Subsequent branching identified a gallo-anserae group and a group containing all other Neognathae. Surprisingly, Columba livia (Columbiforme order) clustered with Galliformes. With represented species, Procellariiformes and possibly Ciconiiformes, and Pelicaniformes were suggested as paraphyletic, in agreement with conclusions from some studies based on mitochondrial DNA sequences.

Cloning and characterization of androgen receptor from bullfrog, Rana catesbeiana

We have cloned and characterized a full-length cDNA of androgen receptor (AR) from the testis of bullfrog, Rana catesbeiana. The cDNA contains an open reading frame of 2328 nucleotides encoding a protein of 776 amino acid residues. The bullfrog AR shows high homology with ARs from other species in its amino acid sequence. Its overall homology with those of African clawed frog, Japanese eel and human is 70, 53, and 63%, respectively. As expected, the N-terminal domain shows much less homology (30-59%) than both DNA-binding domain (85-92%) and ligand binding domain (80-89%). Northern blot analysis detected the bullfrog AR message as a single transcript of around 9 kb only in the testis. However, RT-PCR analysis revealed that AR mRNA is also expressed in other tissues although the levels are very low compared to that in the testis. Western blot analysis of whole tissue extracts showed the presence of AR protein in fore brain, heart, and testis. The AR gene is present as a single copy in bullfrog based on Southern blot analysis of genomic DNA. Altogether, the results suggest that the bullfrog AR is evolutionary conserved and may have functions similar to those shown in other species.

Expression and characterization of Xenopus laevis SRY-related cDNAs, xSox17alpha1, xSox17alpha2, xSox18alpha and xSox18beta

Sox is a large family of genes related to the sex-determining region Y gene (designated as the SRY gene). Sox genes encoding DNA-binding transcriptional factors are found in many animals and are involved in developmental events. In this study, we newly isolated and sequenced novel Sox cDNAs from African clawed frog (Xenopus laevis). Five clones isolated here were classified into four distinct Sox genes designated as xSox17alpha1, xSox17alpha2, xSox18alpha and xSox18beta. All four belong to a subtype of SOX family, type F. The cDNA xSox17alpha1 contains essentially the same nucleotide sequence as that identified as Sox17alpha in a previous work (Cell 91 (1997) 397), whereas xSox17alpha2 is a distinct gene with high homology to xSox17alpha1. The clones, xSox18alpha and xSox18beta, are highly homologous to each other over the entire nucleotide sequences. The xSox18alpha and xSox18beta genes encode 363 and 361 amino acids, respectively. Genomic Southern hybridization analysis showed the existence of two copies of the xSox18. Northern analysis indicated that the xSox18 gene was expressed in the spleen and kidney and the size of the transcript was estimated to be 2.4 knt. Electrophoretic mobility shift assays indicated that recombinant xSox18 polypeptide was capable of binding to the HMG consensus nucleotide sequence, AACAAT.

Expression of amylase and other pancreatic genes in Xenopus

To better understand the relationship between the endocrine and exocrine cell types in the Xenopus pancreas, we have cloned the Xenopus amylase cDNA and compared its expression profile with that of four other pancreatic markers: insulin, glucagon, elastase and trypsinogen. Our results demonstrate that the first pancreatic marker to be expressed is insulin, exclusively in the dorsal pancreas. These insulin-expressing cells form small groups which resemble islets, but no insulin is detected in the ventral pancreas until stage 47. In contrast, the exocrine markers, amylase, elastase and trypsinogen are first expressed only in the ventral pancreas beginning at stage 41; by stage 45 their expression extends into the dorsal pancreas. Glucagon, on the other hand, is not expressed in the pancreas until stage 45. In the endocrine cell clusters we do not find glucagon-expressing cells surrounding insulin-expressing cells, either in the tadpole or in the mature frog pancreas.

Anomalous rates of evolution of pancreatic polypeptide and peptide tyrosine-tyrosine (PYY) in a tetraploid frog, Xenopus laevis (Anura:Pipidae)

The South African clawed frog Xenopus laevis is believed to have arisen as a result of a tetraploidization event occurring approximately 30 million years ago. Two molecular forms of pancreatic polypeptide (PP) have been isolated from an extract of the pancreas of this species and two molecular forms of peptide tyrosine-tyrosine (PYY) from the intestine. Despite the fact that the amino acid sequence of PP has, in general, been very poorly conserved during the evolution of tetrapods (only Pro(5), Pro(8), Gly(9), Ala(12), Tyr(27), Arg(33) and Arg(35) are invariant among species studied so far), the two Xenopus PPs differ by only a single amino acid substition (Asp(22)-->Glu). In contrast the two molecular forms of PYY differ by six amino acid substitutions (Glu(15)-->Gln, Thr(18)-->Ala, Leu(21)-->Met, Ile(22)-->Thr, Ile(28)-->Val, Val(31)-->Ile). The data imply that strong evolutionary pressure is acting to conserve the functional domain in both genes encoding PP and so suggest that PP may have an important physiological role in amphibians (although the nature of this role has yet to be determined). The more rapid mutation of the functional domain in the genes encoding PYY, a peptide whose amino acid sequence has been quite well conserved in tetrapods and whose physiological significance is well established, suggests that one of the PYY genes may be evolving towards a new function or towards becoming a pseudogene.

Development of the pancreas in Xenopus laevis

Xenopus embryos have several experimental advantages for studying development. Although these advantages have traditionally been used to elucidate mechanisms of early development, they can also be exploited to investigate issues later in development such as organogenesis. We have begun to study pancreatic organogenesis in Xenopus. Using histological and molecular marker analysis, we characterized the anatomy of the developing pancreas in Xenopus embryos from the time of initial pancreatic rudiment formation to the time when the tadpole starts to feed. We examined the expression of various endocrine hormones, exocrine gene products, and pancreatic transcription factors. Interestingly, the endocrine hormone insulin has restricted expression in the dorsal pancreas. Investigation of pancreatic specification during gastrulation demonstrates that insulin expression is regionalized along the dorsoventral axis early in development.

Proinsulin cDNAs from the leopard frog, Rana pipiens: evolution of proinsulin processing

We have isolated a proinsulin cDNA from the Amphibian Rana pipiens. The predicted R. pipiens insulin A- and B-chain amino acid sequences differ from that deduced from the closely related Rana catesbeiana at one residue (Asp for Pro at B2). The R. pipiens and Xenopus laevis proinsulin precursor sequences are of identical length, with the amino acid sequences of the mature A- and B-chains being well conserved. The proinsulin C-peptide amino acid sequence is less well conserved between R. pipiens and X. laevis and also differs in length. The R. pipiens C-peptide is shorter than the homologous X. laevis sequence due to a two amino acid residue truncation. The truncation of the R. pipiens C-peptide compensates for a two amino acid residue extension observed at the N-terminal of the A-chains of insulins from Ranid frogs. A change in the site of proinsulin processing can explain both the C-peptide and A-chain length differences. The evolution of the new proinsulin processing site required two amino acid substitutions.

Molecular cloning and expression of Xenopus laevis requiem cDNA1

Requiem is an apoptosis-associated gene, which was originally identified in mouse (T.G. Gabig et al., J. Biol. Chem. 269 (1994) 29515-29519). In this study, we isolated five independent cDNA clones for frog Requiem (xReq) from Xenopus laevis ovary. Sequence analysis of the multiple cDNAs has suggested that Xenopus genome contains at least two non-allelic copies of the xReq gene. The amino acid sequence deduced from the cDNAs contained a single Krüppel-type zinc-finger motif and two PHD-finger motifs. Northern analysis revealed that the ovary expressed xReq transcripts with different sizes.

Molecular cloning and characterization of Xenopus insulin-like growth factor-1 receptor: its role in mediating insulin-induced Xenopus oocyte maturation and expression during embryogenesis

We have cloned a complementary DNA encoding the putative Xenopus insulin-like growth factor-1 (xIGF-1) receptor. Injection of messenger RNA derived from the cloned complementary DNA into Xenopus oocytes resulted in the expression and correct processing of the receptor's alpha- and beta-subunits. Using antibodies generated against protein expressed against the cloned sequence, we demonstrated that the endogenous xIGF-1 receptor in Xenopus oocytes was activated by nanomolar concentrations of mammalian IGF-1 and by insulin approximately 100-fold higher in concentration. This receptor activation profile correlated with hormone-induced Xenopus oocyte maturation. Furthermore, injection of a neutralizing antiinsulin receptor antibody into Xenopus oocytes inhibited hormone-induced xIGF-1 receptor activation. These results provide molecular and biochemical evidence supporting a role for xIGF-1 receptor in mediating insulin/IGF-1-induced Xenopus oocyte maturation. We also report here that embryonic transcription of xIGF-1 receptor is activated during the formation of the central nervous system in early Xenopus embryos.

Elephantfish proinsulin possesses a monobasic processing site

Total pancreatic RNA from the holocephalan species Callorhyncus milii (elephantfish) was used to make cDNA as a template for the polymerase chain reaction. Three redundant primers based on the known amino acid sequence of elephantfish insulin were used to amplify a fragment of proinsulin comprising truncated B-chain, complete C-peptide, and complete A-chain. Whereas the C-peptide/A-chain junction contained the expected dibasic cleavage site (-Lys-Arg-), the B-chain/C-peptide junction was found to contain only a single Arg, the first such site to be unequivocally associated with the proteolytic processing of a proinsulin to insulin. Examination of the flanking sequences around this site shows that a typical endocrine/neuroendocrine PC3 conversion enzyme should still be able to cleave, as the general requirements for precursor processing at a monobasic site are satisfied, notably a basic residue (Lys) at the -4 position. An acidic residue (in this case Asp) at the +1 position, which is seen in all known proinsulins, is maintained. The corresponding genomic DNA fragment of elephantfish proinsulin was also amplified by PCR, revealing a 402-bp intron at the conserved IVS-2 position within the C7 codon.

XLS13A and XLS13B: SRY-related genes of Xenopus laevis

SRY-related cDNAs, XLS13A and XLS13B, have been isolated from Xenopus laevis ovary. The cDNAs encode polypeptides of 382 and 375 amino acids, respectively. Nucleotide sequences of the two cDNAs are highly homologous to each other. The type-A and type-B XLS13 proteins, and xSox13 reported previously share an identical high mobility group (HMG) box at the amino acid level, although they contain silent nucleotide alterations. The HMG box exhibits strong similarity (> 93% amino acid identity) to those of mouse Sox4/human SOX4 and chicken Sox11/human SOX11. The size of XLS13A/XLS13B mRNA was estimated to be 2.8 knt in Xenopus ovary by Northern analysis. Reverse transcription/polymerase chain reaction (RT/PCR) assay indicated that XLS13A and XLS13B mRNAs are present in various tissues of adult frog. The mRNAs of XLS13A and XLS13B of maternal origin found in unfertilized eggs disappear in the early stages of the Xenopus embryo. DNA-binding properties of the XLS13 HMG domain were examined by electrophoretic mobility shift assay (EMSA). The HMG domain preferentially binds to the canonical target sequence of SOX proteins, AACAAT, in vitro.

TGF-beta signals and a pattern in Xenopus laevis endodermal development

We have analyzed two gene products expressed in the early endoderm of Xenopus laevis: Xlhbox-8, a pancreas-specific transcription factor and intestinal fatty acid binding protein (IFABP), a marker of small intestinal epithelium. Expression of the pancreas marker relies on cell signaling mediated by both the TGF-beta and FGF classes of secreted peptide growth factors, whereas, expression of the more posterior small intestinal marker does not. Endodermal explants devoid of mesoderm express both markers in a regionalized manner. Cortical rotation is required for the expression of the more anterior marker, Xlhbox-8, but not for the small intestinal marker, IFABP. These findings suggest that endodermal patterning is dependent, in part, on the same events and signals known to play important roles in mesodermal development. Furthermore, inhibition of TGF-beta signaling in the endoderm leads to ectopic expression of both mesodermal and ectodermal markers, suggesting the TGF-beta signaling may play a general role in the segregation of the three embryonic germ layers.

Cloning and expression of a Xenopus liver cDNA encoding a fructose-phosphate-insensitive regulatory protein of glucokinase

Xenopus liver contains a protein inhibitor of glucokinase that, in contrast to the mammalian regulatory protein of glucokinase, is insensitive to fructose 6-phosphate and fructose 1-phosphate [Vandercammen A. & Van Schaftingen, E. (1993) Biochem. J. 294, 551-556]. The purpose of this work was to compare the primary structure and other properties of this Xenopus protein with those of its rat liver counterpart. A Xenopus laevis liver cDNA library was screened using the cDNA encoding the rat liver regulatory protein as a probe. The cloned cDNA was 2534 bp long and encoded a 619-amino-acid protein with a molecular mass of 68695 Da and 57% identity with the rat liver regulatory protein. This identity was only about 30% in an internal region (amino acids 349-381) and in the 70 carboxy terminal-residues. The Xenopus cDNA was expressed in Escherichia coli and the recombinant regulatory protein was purified to near homogeneity and found to have the same size, reactivity to antibodies and effects on the kinetics of glucokinase as the protein purified from Xenopus liver. In contrast to the rat liver regulatory protein, both recombinant and native Xenopus regulatory proteins were insensitive to fructose 6-phosphate, fructose 1-phosphate and to physiological concentrations of Pi, and they inhibited Xenopus glucokinase with greater affinity than rat glucokinase. These results allow one to conclude that the fructose-phosphate-insensitive protein of lower vertebrates is homologous to the fructose-6-phosphate-sensitive and fructose-1-phosphate-sensitive protein found in mammals.

Phylogeny of the insulin-like growth factors (IGFs) and receptors: a molecular approach

The IGFs (IGF-I and IGF-II) are essential for normal mammalian growth and development. Their actions are mediated primarily by their interactions with the type I IGF receptor (IGF-I receptor), a transmembrane tyrosine kinase. The ligands and the IGF-I receptor are structurally related to insulin and to the insulin receptor, respectively. Analysis of evolutionary conservation has often provided insights into essential regions of molecules such as hormones and their receptors. The genes for insulin and IGFs have been partially characterized in a number of vertebrate species extending evolutionarily from humans as far back as fish. The sequences of the exons encoding the mature insulin and IGF peptides are highly conserved among vertebrate species, and IGF-I-like molecules are found in species whose origins extend back as much as 550 million years. The insulin receptor is also highly conserved in vertebrate species, and an insulin-receptor-like molecule has been characterized in Drosophila. In contrast, IGF-I receptors have only been characterized in mammalian species and partially studied in Xenopus, in which the tyrosine kinase domain is highly conserved. Studies are presently being undertaken to analyze in more detail the regulation of the genes encoding this important family of growth factors and the structure/function relationships in the gene products themselves.

Ligase-free subcloning: a versatile method to subclone polymerase chain reaction (PCR) products in a single day

Often, it is convenient to subclone polymerase chain reaction (PCR) products into a plasmid vector for subsequent replication in bacteria, but conventional subcloning methods often fail. We report a rapid and versatile method to subclone PCR products directionally into a specific site of virtually any plasmid vector. The procedure requires only four primers, does not require DNA ligase, and may be accomplished in a single day. Ligase-free subcloning is performed by incorporating into the PCR primers sequences at the 5' ends that result in PCR products whose 3' ends are complementary to the 3' ends of the recipient linearized plasmid. The PCR product and the linearized plasmid are spliced together in a second PCR reaction in which Taq polymerase extends the complementary overlapping 3' ends (ligation by overlap extension). Denaturation followed by heterologous reannealing and cyclization results in a cyclic recombinant plasmid with two nicks that may be used directly to transform competent Escherichia coli. In our hands, ligase-free subcloning is rapid, and offers many advantages over existing strategies.

RNA template-specific polymerase chain reaction (RS-PCR): a novel strategy to reduce dramatically false positives

We report a novel modification of the reverse transcription-polymerase chain reaction method that we have dubbed RNA template-specific PCR (RS-PCR). With this approach, the 5' end of the first strand is tagged with a unique nucleotide sequence during reverse transcription which may then be exploited to amplify preferentially RNA-derived sequences. In our hands, RS-PCR greatly reduces the frequency of false positives and virtually eliminates carryover contamination from DNA fragments amplified in previous experiments.

Evidence that Xenopus laevis contains two different nonallelic insulin-like growth factor-I genes

Using the polymerase chain reaction (PCR), we have amplified and characterized partial nucleotide sequences of two distinct insulin-like growth factor-I genes (designated IGF-I' and IGF-I") from the amphibian, Xenopus laevis. The amplified fragments encoded much of the coding region of the mature peptide (exon III in mammalian IGF-I genes), and exhibited 93% similarity to each other, and 68-82% similarity to mammalian IGF-I amino acid sequences. Southern blot analysis using genomic DNA from a homozygous frog revealed that these two genes are nonallelic in a single organism, like the two nonallelic genes encoding Xenopus insulins that we have characterized previously. Furthermore, both IGF-I mRNAs are expressed in similar quantities in adult liver.