Minor Transcripts Research Articles

Methylation of chick UbI and UbII polyubiquitin genes and their differential expression during spermatogenesis

Northern analysis demonstrated that levels of ubiquitin transcript increased during the chicken testis maturation process, in agreement with the previously published increase of ubiquitin during this differentiation process. Specific probes for four different ubiquitin genes (two polyubiquitins, UbI and UbII, and two ubiquitin-fusion genes, UbCep52 and UbCep80) allowed us to analyse the expression of each individual gene. UbI polyubiquitin gene was expressed in all the tissues tested, and its transcript was the most abundant ubiquitin RNA in all of them. Unspliced UbI transcript, already detected in stressed chicken-embryo fibroblast, was also present in immature testis and reticulocytes. UbII, a chicken polyubiquitin gene not previously found expressed and not heat-shock-inducible, was specifically stimulated during the testis maturation process. Two minor ubiquitin fusion transcripts of 0.6 and 0.7 kb, corresponding to UbCep52 and UbCep80 respectively, were also found in chicken testis. Although differentially expressed, it was found that UbI and UbII chicken polyubiquitin genes had an HTF ('HpaII tiny fragments') island (CpG-rich and constitutively unmethylated region) in their 5' proximal non-coding region. In addition, we demonstrated the coexistence of 3' and/or 5' relatively distal methylated sites together with these 5' proximal HTF islands in both chicken polyubiquitin genes. 3' and 5' distal UbI CCGG sites were specifically hypermethylated in mature testis, whereas a 3' distal UbII CCGG site was found to be about 50% methylated in all DNAs tested.

Expression of the human papillomavirus type 16 genome in SK-v cells, a line derived from a vulvar intraepithelial neoplasia

The SK-v cells, established from a premalignant vulvar lesion, contain human papillomavirus type 16 (HPV-16) sequences integrated at a single cellular site and derive from a cell clone present in vivo. Transcription of the HPV-16 genome in SK-v cells was analysed by cDNA heteroduplex mapping and sequencing, and by RNase mapping. Viral sequences were shown to be transcribed into virus-cell fusion messengers. The two major transcripts have a coding capacity for a truncated E6 protein, an E7 protein and an E1-E4 fusion protein, but differ in their 3' virus-cell junction. Minor transcripts have a coding capacity for a full-length E6 protein and another truncated version of E6. The transcription pattern in the E6-E7 region was found to be the same both in SK-v cells and in CaSki cells, a line derived from an invasive cervical carcinoma. Immunoprecipitation experiments showed that the E6 protein (18K) and, predominantly, the E7 protein (20K) are expressed in SK-v cells as in CaSki cells. The E7 protein was found in a two- to threefold lower amount in SK-v cells, but showing the same half-life (about 1 h).

Cloning and Characterization of a Disease Resistance Response Gene in Pea Inducible byFusarium solani

Disease resistance response genes (DRRG) of peas are expressed as the tissue is expressing race-specific or nonhost resistance. A pea genomic clone DRRG49-c encompassing one DRRG structural gene, the expression of which is correlated with the expression of disease resistance, was sequenced and characterized. The 2.3-kb genomic segment sequenced encompassed 986 bp 5' to the major transcriptional initiation site, a 474-bp open-reading frame interrupted by one 88-bp AT-rich intron and an additional 574-bp segment 3' from the stop codon. Southern blot analysis indicated that the DRRG structural gene is one of a multigenic family, and an estimated five copies exist within the pea genome. Primer extension analysis of the 5' terminus of the corresponding RNA suggested the presence of one major transcript with possibly two minor transcripts. The major transcript, located 65 bp from the translational initiation site, was expressed when challenged with Fusarium solani f. sp. phaseoli but not with water. The structural gene sequence corresponding to the genomic clone DRRG49-c is not identical with the structural gene of the cDNA clone DRRG49-a used as a probe for northern blot analysis, and thus, a possibility remains that it is not expressed in peas; however, the DRRG49-c promoter was able to express the chloramphenicol acetyltransferase reporter gene in tobacco protoplasts. Western blot analysis using antiserum prepared from a beta-galactosidase-DRRG49-a fusion protein identified the DRRG49 gene product as a major protein accumulating during the host-pathogen interactions.

Reiterative copying byE.coliRNA polymerase during transcription initiation of mutant pBR322 tet promoters

The major in vitro transcripts from the tet promoter of pBR322 derivatives pTA22 and pTA33 have heterogeneous 5' ends consisting of variable lengths of oligo(A). Their structure is 5'pppAnU..., where n ranges from 1 to greater than 12, but the template strand can encode at most four A residues at the site of transcription initiation. The abundance of additional A residues at the 5' end of the pTA22 and pTA33 tet transcripts could be reduced by elevating the concentration of UTP, but even at high concentrations (greater than 1 mM) non-cognate A residues were still observed. Aberrant initiation was not artifactual since the major and minor transcripts of the pBR322 tet promoter region, and other transcripts arising from minor promoters on pTA22 or pTA33 DNA all had unique 5' termini. Mixing experiments showed that RNA polymerase did not utilize pppA2-4-OH produced by abortive initiation as primers. The data suggest that the initial nascent RNA chain 'slips' in the 5' direction during elongation opposite T4 on the template strand causing RNA polymerase to reiteratively add A residues to the 5' end of the transcript. The generality and possible significance of this mechanism is discussed.

Regulation of transcription and promoter mapping of the structural genes for nitrogenase (nifHDK) of Azospirillum brasilense Sp7.

Transcription of the structural genes for nitrogenase (nifHDK) in Azospirillum brasilense Sp7 was analysed using Northern blots of total RNA extracted from cultures grown under nitrogen-fixing conditions. Hybridization with an internal nifH probe revealed two transcripts, a major one (by concentration) of 1.1 kb corresponding to nifH and a minor one of 5.6 kb corresponding to nifHDK. Hybridization with nifD or nifK probes revealed the minor transcript of 5.6 kb. This confirms that the nifHDK genes are organized as a single transcription unit and suggests regulation at the level of termination of transcription. The complete nucleotide sequence of nifH was established and the DNA region upstream of the initiation codon was analysed for transcription and translation signals. The nifH open reading frame (ORF) is preceded by an NtrA-dependent promoter and two elements homologous to upstream activator sequences (UAS) required for NifA-mediated activation in other diazotrophs. Promoter mapping with S1 nuclease revealed two start sites located 10 bp and 40 bp downstream of the NtrA-dependent promoter.

Alternative splicing of β-galactosidase mRNA generates the classic lysosomal enzyme and a β-galactosidase-related protein

We have isolated two cDNAs encoding human lysosomal beta-galactosidase, the enzyme deficient in GM1-gangliosidosis and Morquio B syndrome, and a beta-galactosidase-related protein. In total RNA from normal fibroblasts a major mRNA of about 2.5 kilobases (kb) is recognized by cDNA probes. A minor transcript of about 2.0 kb is visible only in immunoselected polysomal RNA. A heterogeneous pattern of expression of the 2.5-kb beta-galactosidase transcript is observed in fibroblasts from different GM1-gangliosidosis patients. The nucleotide sequences of the two cDNAs are extensively colinear. However, the short cDNA misses two noncontiguous protein-encoding regions (1 and 2) present in the long cDNA. The exclusion of region 1 in the short molecule introduces a frameshift in its 3'-flanking sequence, which is restored by the exclusion of region 2. These findings imply the existence of two mRNA templates, which are read in a different frame only in the nucleotide stretch between regions 1 and 2. Sequence analysis of genomic exons of the beta-galactosidase gene shows that the short mRNA is generated by alternative splicing. The long and short cDNAs direct the synthesis in COS-1 cells of beta-galactosidase polypeptides of 85 and 68 kDa, respectively. Only the long protein is catalytically active under the assay conditions used, and it is capable of correcting beta-galactosidase activity after endocytosis by GM1-gangliosidosis fibroblasts. The subcellular localization of cDNA-encoded beta-galactosidase and beta-galactosidase-related proteins is different.

Spatial and temporal distribution of bovine herpesvirus 1 transcripts

Northern (RNA) blot analysis was used to determine the spatial and temporal distribution of bovine herpesvirus 1 (BHV-1) transcripts. Total RNA was isolated from Madin-Darby bovine kidney cells which had been infected with BHV-1.2b strain K22 or BHV-1.1 strain Jura in the presence or absence of metabolic inhibitors. Cloned restriction fragments representing the entire genome of strain K22 were labeled with 32P and hybridized to immobilized RNA. A total of 54 BHV-1 transcripts were found, ranging in size from 0.4 to larger than 8 kilobases (kb). The inverted repeat regions and an adjacent segment of the unique large part of the BHV-1 genome encoded three major immediate-early (IE) transcripts and one minor IE transcript enriched after cycloheximide treatment of infected cells. Late transcripts were identified by drastically reduced abundance after cytosine arabinoside (araC) treatment. Twelve late transcripts were encoded mainly by the unique long genome region, with a cluster of four transcripts located on HindIII fragment K (map units 0.677 to 0.733). The 21 transcripts unaffected by araC treatment were defined as early; they showed dispersed locations over the whole genome, with a cluster on the unique short sequence. The 17 remaining transcripts could not be classified unambiguously as early or late by these techniques. The IE transcript with a size of 4.2 kb exhibited homology with the single IE gene of pseudorabies virus, and the IE transcript with a size of 2.9 kb was encoded in part by the genome region known to be transcriptionally active during latency.

Molecular genetics of pecanex, a maternal-effect neurogenic locus of Drosophila melanogaster that potentially encodes a large transmembrane protein

In the absence of maternal expression of the pecanex gene, the embryo develops severe hyperneuralization similar to that characteristic of Notch mutant embryos. We have extended a previous molecular analysis of the chromosomal interval that encompasses pecanex by using additional deficiencies to localize the locus on the molecular map. RNA blot analysis shows that the locus encodes a rare 9-kb transcript as well as minor transcripts of 3.7 and 2.3 kb. The temporal expression of these transcripts is appropriate for a neurogenic locus. Phenocopies of the mutant phenotype have been produced following microinjection of antisense RNA corresponding to a portion of the pecanex transcripts. Conceptual translation of a partial coding sequence compiled from cDNA and genomic clones indicates that the pecanex locus potentially encodes a large, membrane-spanning protein.

The PET54 gene of Saccharomyces cerevisiae: characterization of a nuclear gene encoding a mitochondrial translational activator and subcellular localization of its product.

The product of the nuclear Saccharomyces cerevisiae gene PET54 is specifically required, along with at least two other nuclear gene products, for translation of the mitochondrial mRNA encoding subunit III of cytochrome c oxidase (coxIII). We have genetically mapped PET54 (to the right arm of chromosome VII, 4.8 cM centromere-distal to SUF15), and have biochemically characterized the gene and its product. We determined the nucleotide sequence of a 1.6-kb DNA fragment carrying PET54 and identified the PET54 reading frame by determining the sequence of an ochre mutant allele as well as frameshift and frameshift-revertant alleles of the gene. The wild-type PET54 gene encodes a slightly basic 293-amino acid protein. PET54 is expressed from two mRNAs, both with unusual features: a major transcript with an extremely short 5'-untranslated leader, and a minor transcript with a relatively long 5'-leader carrying three short open reading frames. Antiserum raised against a trpE-PET54 fusion protein was used to probe subcellular fractions. These experiments showed that the PET54 protein is specifically associated with mitochondria, suggesting that it is likely to act directly in coxIII translation.

Insertion of the SfMNPV polyhedrin gene into an AcMNPV polyhedrin deletion mutant during viral infection

The Autographa cafifornica nuclear polyhedrosis virus (AcMNPV) polyhedrin deletion mutant, d10A, was cotransfected with the recombinant plasmid SfMNPV· HindIII-L, which contains the Spodoptera frugiperda nuclear polyhedrosis virus (SfMNPV) polyhedrin gene. An occlusion-positive hybrid virus was obtained which contained a DNA insertion at approximately 13 map units on the AcMNPV·d10A genome. A fine structure restriction map of cloned viral DNA fragments from this region revealed that most of the 1.7-kb AcMNPV· PstI-O fragment (13.4–14.7 map units) had been replaced with a 3.7-kb fragment, containing the SfMNPV polyhedrin gene with flanking sequences and the entire pUC8 plasmid. Subclones of this region were sequenced and the exact site of insertion was determined. Flanking the insert were 43 and 60 by of unknown origin at the 5′ and 3′ ends, respectively. Transcription was examined in the region of the insertion in both wild-type AcMNPV and the AcMNPV/SfMNPV (Ac/Sf) hybrid. In the AcMNPV, a nested set of seven overlapping transcripts ranging from 2.2 to 5.3 kb was found, each with coterminal 3′ ends. Only the 2.2-kb transcript was found to be expressed early and throughout infection. The SfMNPV insertion interrupted this transcriptional unit and produced a more complex pattern of transcription. Alterations included a nested set of three overlapping transcripts with coterminal 5′ ends, including the SfMNPV polyhedrin mRNA, transcripts originating in AcMNPV and terminating in either SfMNPV or pUC8 sequences, and other minor transcripts. The SfMNPV polyhedrin gene was sequenced and the locations of the 5′ and 3′ ends of polyhedrin mRNA were mapped. An analysis of SfMNPV polyhedrin protein expression showed that the SfMNPV polyhedrin gene in the hybrid virus was expressed at approximately one-fourth the level of the wild-type AcMNPV polyhedrin gene. Expression of a β-galactosidase gene under the control of the SfMNPV polyhedrin promoter in the AcMNPV·d10A mutant was also investigated.

Sense and antisense transcripts of traM, a conjugal transfer gene of the antibiotic resistance plasmid R100

The region of the antibiotic resistance plasmid R100 that encodes the plasmid-specific transfer gene traM has two tandemly aligned promoters separated by 145 nucleotides. The principal transcripts are 705 and 562 nucleotides long. Minor transcripts are 1550 and 1700 nucleotides long. The 705-base transcript appears to encode an 11 kD traM protein. The 562-base transcript does not encode a detectable protein. When subcloned on short fragments, the promoter for the 562-base transcript initiates efficiently but that for the 705 site does not. The 3' ends of the 705 and 562 base transcripts end inside the traJ ORF. Thus they provide additional sense RNA to compete with traJ for finP, the antisense translational regulator of traJ. A model is proposed for the participation of these sense and antisense transcripts in the control of expression of the traJ gene.

Transcription of rat aldolase B gene: Minor RNAs are transcribed by RNA polymerase III

Rat aldolase B gene was confirmed to have multiple transcriptional initiation sites; one site for the major transcript and at least two sites for the minor transcripts which are located at about 30 to 60 bases upstream of the major site. Although the transcription of the minor transcripts was entirely resistant to 1 μg/ml of α-amanitin, which inhibits RNA polymerase II, it was completely blocked by 1000 μg/ml of the drug which inhibits RNA polymerase III. The ☐ A- and B-like sequences, which are known to exist in the internal split promoter region of class III genes, were also found to be located around the initiation sites of two minor transcripts from the rat aldolase B gene. Analyses using deletion mutants of the promoter region of the gene suggested that the synthesis of the minor transcripts depends on the ☐ A- and B-like sequences, but does not depend on a TATA promoter element. Northern blot analysis indicated that the minor transcripts are only a few percent of the total transcripts derived from the aldolase B gene. Some of the minor transcripts analyzed were shown to have a mature size in the poly(A) + RNA fraction. Thus, the minor transcripts of rat aldolase B gene are apparently transcribed by RNA polymerase III, and subsequently normally processed and polyadenylated. The possible function of these minor transcripts will be discussed.

The azurin gene from Pseudomonas aeruginosa

We have cloned and sequenced the Pseudomonas aeruginosa azurin structural gene and its flanking regions. The DNA sequence predicts a pre-protein with a signal peptide of 19 amino acids followed by the 128-amino-acid mature azurin protein. Nuclease-S1 mapping and primer elongation experiments indicated two 5' termini of the azurin transcript. The major transcript of the azurin gene is initiated around 35 base pairs upstream from the translational start. The minor transcript, with a promoter region sharing homology with a consensus nif promoter of Klebsiella pneumoniae and also with other Pseudomonas genes, is initiated 145 base pairs upstreams of the azurin initiation codon. Downstream from the azurin structural gene a sequence similar to a transcriptional terminator is found. Northern blot analysis indicated two sizes of the azurin mRNA (0.54 kb and 0.65 kb) confirming the S1 mapping and the predictions from the nucleotide sequence.

Ubiquftin mRNA is a major stress-induced transcript in mammalian cells

Ubiquitin mRNA was found to be an abundant transcript which was induced by heat shock (HS), and certain other stresses in mammalian cells. In Chinese hamster cells, the 2 major ubiquitin transcripts of 2.6 kb and 1.7 kb were induced coordinately, while a minor ubiquitin transcript of 0.8 kb was not induced; the response was similar in human cells with induction of the 2.5 kb Ub C and 1.0 kb Ub B transcripts. A representative ubiquitin cDNA clone, isolated from a cDNA library derived from HS-treated Chinese hamster cells, coded for a typical tandem repeat polyubiquitin transcript. Only a portion of the 5' nontranslated sequence of this clone had homology with the previously published corresponding region in human Ub B mRNA. Oligonucleotide probes complementary to the portion of the 5' nontranslated sequence with homology to the human sequence and also portions with no homology hybridized only to the 1.7 kb transcript. There was coordinate induction of ubiquitin, HSP27, and HSP70 mRNA by HS as determined by both increased RNA and increased transcription. Ubiquitin mRNA was induced by certain DNA damaging agents, in particular the alkylating agent methylmethane sulfonate, or incubation in isoleucine-deficient medium under conditions where the other HSP mRNA were not.

Mapping of two novel transcripts of bovine papillomavirus type 4.

A cDNA library was synthesized using RNA from bovine papillomavirus type 4 (BPV-4)-induced papillomas. The major viral transcript was characterized by sequencing of its cDNA, primer extension mapping and S1 nuclease protection studies. The transcript initiates at multiple sites between nucleotide (nt) 777 and nt 902, contains a splice junction between nt 1016 and nt 3376 and terminates at nt 4034, using the early polyadenylation signal at nt 4009. Sequencing of viral cDNAs and mRNA revealed deviations from the reported genomic sequence between nt 3412 and nt 3460. These resulted in a temporary frameshift, abolished the translation termination codon of the E5a open reading frame (ORF), and introduced a new termination codon in the E4 ORF. The new uninterrupted E5 ORF was found to have greater homology to the E4 ORFs of other papillomaviruses than to their E5 ORFs. The E5 ORF of BPV-4 is joined to a five codon ORF in the leader exon of the major transcript, in the same way as the E4 ORF in the major transcripts of BPV-1 and human papillomavirus type 11. On this basis, it has been redesignated E4, and the viral genomic sequence revised. Two novel transcriptional promoters of BPV-4 were defined: a putative controller of the multiple major RNA start sites around nt 870 and a minor TATA box at nt 691. In addition, minor early region transcripts were mapped which initiate between nt 3071 and nt 3152. None of these transcripts utilizes the splice site at nt 3376. These messages may express E2-encoded functions.

Structure and neural expression of a zebrafish homeobox sequence

A genomic library of zebrafish was constructed and screened with homeobox-containing probes. One of the positive clones contains a transcribed region which shares extensive sequence homology with the murine Hox-1.4 and Hox-2.6 genes and the human HHO.c13 gene. Characterization of this zebrafish homologue (ZF-13) with respect to expression demonstrated that it is transcribed during embryogenesis where a major RNA species of 2.5 kb and a minor transcript of 4.6 kb are detected. The highest concentration of both transcripts was found in embryos at the stage of somite formation. By in situ hybridization the spatial localization of expression was analysed in hatching embryos. Hybridization signals were mainly detected throughout the neural tube and in the brain. A small amount of RNA derived from ZF-13 was localized in differentiated muscle cells. Our results suggest that homeobox genes of distantly related vertebrate species are very similar with respect to structure and function.

Differential expression of the homeobox gene Hox-1.3 in F9 embryonal carcinoma cells.

The Hox-1.3 gene is located on mouse chromosome 6 and has been previously shown to be expressed in mouse embryos and adults. In this study, we have examined the steady-state levels of the Hox-1.3 transcripts in undifferentiated and differentiated F9 embryonal carcinoma cells. We find that there is a rapid increase of Hox-1.3 transcripts after differentiation induction of F9 cells. The level of the major 1.85-kilobase (kb) transcript peaks at 16-24 hr after differentiation induction of F9 cells. By using primer extension techniques the 5' ends of the major 1.85-kb transcript have been mapped to two sites in induced F9 cells. Cellular fractionation of RNA and transfer blot gel analysis has localized one minor transcript to the nucleus, whereas the major transcript and two additional minor transcripts appear in the nucleus and the cytoplasm of induced F9 cells. The results of nuclear run-off experiments with uninduced and induced F9 cell nuclei indicate that there is a substantial increase in the rate of Hox-1.3 transcription upon induction of F9 cells with retinoic acid.

Transcriptional organization of the phycocyanin subunit gene clusters of the cyanobacterium Anacystis nidulans UTEX 625

The phycocyanin subunit gene cluster is duplicated on the chromosome of the cyanobacterium Anacystis nidulans UTEX 625. The two gene clusters cpcB1A1 (left) and cpcB2A2 (right) are separated by about 2,500 base pairs, and in each cluster the beta-subunit gene is located upstream from the alpha-subunit gene. Filter hybridizations with phycocyanin-specific probes to total RNA detected at least two major transcripts that were 1,300 to 1,400 nucleotides long. Besides these major mRNA species, two minor transcripts of 3,400 and 3,700 nucleotides covering one of the gene clusters and the region between the clusters were found. No additional minor transcripts were found in the intergenic region between the two phycocyanin gene clusters. The lengths of the major mRNAs indicated that the beta- and alpha-subunit genes were cotranscribed. No apparent homologies were found when the DNA sequences located upstream from the proposed ribosome-binding site of the two phycocyanin beta-subunit genes were compared. Northern hybridizations with gene cluster-specific probes from the regions 5' of the beta-subunit genes, as well as S1 nuclease mapping and mRNA primer extension experiments, showed that both gene clusters were transcribed. The minor transcripts were found to initiate upstream from the left gene cluster. Two mRNA 5' ends were mapped upstream from the cpcB1A1 gene cluster, while only one 5' end was mapped in front of the cpcB2A2 gene cluster. All transcripts were present in RNA preparations from cultures grown under high levels of white light as well as under low levels of red light. The level of phycocyanin-specific mRNA, measured as part of the total RNA, was lower under low levels of red light compared with that under high levels of white light. Conserved sequence motifs were found when the promoter region of the cpcB1A1 gene cluster and promoter regions from other cyanobacterial photosynthesis genes were compared. The DNA sequences covering the proposed transcriptional attenuators and transcriptional stop signals contained several potential hairpin structures. One potential hairpin structure was located immediately downstream of the left phycocyanin gene cluster and was concluded to limit the level of transcription for the minor transcripts initiating upstream of the cpcB1A1 gene cluster.

Analysis of the transcript of the herpes simplex virus DNA polymerase gene provides evidence that polymerase expression is inefficient at the level of translation.

We have mapped the termini and determined the relative abundance and ribosome density of the major cytoplasmic transcript of the DNA polymerase (pol) gene of herpes simplex virus type 1. Nuclease protection and primer extension analyses located the 5' end of the major pol transcript at two closely spaced sites 51 and 57 nucleotides to the left of a BamHI site at map position 0.413. S1-sensitive sites corresponding to additional minor transcripts were found to map further upstream within a palindromic sequence that contains a viral replication origin. The major 3' end was found to map 90 nucleotides upstream of a KpnI site at map position 0.439. Quantitative S1 nuclease assays revealed that pol transcripts were nearly as abundant as transcripts encoded by the viral thymidine kinase gene. However, relatively few pol transcripts were found on large polysomes at 5.5 h after infection, when pol transcripts were most abundant. This was in marked contrast to the polyribosome distribution of transcripts from the thymidine kinase gene and the major DNA-binding protein gene. These results and sequence features of the pol transcript suggest that pol expression is regulated, in part, at the level of translation.

Cloning and expression of a cDNA coding for the human platelet-derived growth factor receptor: evidence for more than one receptor class.

The complete nucleotide sequence of a cDNA encoding the human platelet-derived growth factor (PDGF) receptor is presented. The cDNA contains an open reading frame that codes for a protein of 1106 amino acids. Comparison to the mouse PDGF receptor reveals an overall amino acid sequence identity of 86%. This sequence identity rises to 98% in the cytoplasmic split tyrosine kinase domain. RNA blot hybridization analysis of poly(A)+ RNA from human dermal fibroblasts detects a major (approximately 5.7 kb) and a minor (approximately 4.8 kb) transcript using the cDNA as a probe. Baby hamster kidney cells, transfected with an expression vector containing the receptor cDNA, express an approximately equal to 190-kDa cell surface protein that is recognized by an anti-human PDGF receptor antibody. The recombinant PDGF receptor is functional in the transfected baby hamster kidney cells as demonstrated by ligand-induced phosphorylation of the receptor. Binding properties of the recombinant PDGF receptor were also assessed with pure preparations of BB and AB isoforms of PDGF (i.e., PDGF dimers composed of two B chains or an A and a B chain). Unlike human dermal fibroblasts, which bind both isoforms with high affinity, the transfected baby hamster kidney cells bind only the BB isoform of PDGF with high affinity. This observation is consistent with the existence of more than one PDGF receptor class.