A genomic sequence and cloned complementary DNA has been identified for a novel receptor-like gene of the PDGF receptor/CSF1 receptor subfamily (platelet-derived growth factor receptor/colony-stimulating factor type 1 receptor). The gene recognized a 6.4-kilobase transcript that was coexpressed in normal human tissues with the 5.3-kilobase PDGF receptor messenger RNA. Introduction of complementary DNA of the novel gene into COS-1 cells led to expression of proteins that were specifically detected with antiserum directed against a predicted peptide. When the new gene was transfected into COS-1 cells, a characteristic pattern of binding of the PDGF isoforms was observed, which was different from the pattern observed with the known PDGF receptor. Tyrosine phosphorylation of the receptor in response to the PDGF isoforms was also different from the known receptor. The new PDGF receptor gene was localized to chromosome 4q11-4q12. The existence of genes encoding two PDGF receptors that interact in a distinct manner with three different PDGF isoforms likely confers considerable regulatory flexibility in the functional responses to PDGF.

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To understand the morphogenetic and physiological processes occurring during plant embryogenesis, we isolated cDNA clones homologous to genes preferentially expressed during somatic embryogenesis. One of these cDNA clones detected an embryo-specific mRNA species with a corresponding protein of 66 kDa. The expression pattern of the mRNA is similar between somatic and zygotic embryos of carrots. To characterize the gene encoding this mRNA, we isolated the corresponding genomic clones. Molecular analysis of the DNA from several haploid and diploid carrots showed that the mRNA was encoded by a single copy gene, named DC 8. DNA sequence analysis showed that the gene consisted of three exons and coded for a hydrophilic protein with a central region composed of 17 repeats. At the NH2-terminus no typical signal sequence was found. Immunocytochemical analysis localized the protein primarily in the vacuoles and protein bodies of zygotic embryos; the cytoplasm showed some antibody staining. The protein was also found in cell walls of endosperm tissue. The amount of DC 8 protein was too low for it to be categorized as a seed storage protein; its role in embryo-genesis remains to be determined.

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Polyoma virus minichromosomes were isolated from infected 3T6 cells by hypotonic extraction of isolated nuclei. The kinetics of in vitro DNA synthesis in the nuclear extract was similar to that observed with intact nuclei. The majority of the products of in vitro DNA synthesis sedimented with replicative intermediate (RI) minichromosomes and migrated as two bands (RI-a and RI-b) on 1.4% agarose gels. The kinetics of deoxynucleotide monophosphate incorporation into these species was consistent with the existence of several rate-limiting steps in in vitro replication by polyoma minichromosomes. Electron microscope analysis showed that the RI-a band consisted almost entirely of RI theta structures ranging from 46 to 87% replicated, with one-half of all theta structures 67 +/- 4% replicated. The RI-b material was more complex, consisting of sigma and alpha structures with tails ranging from 7 to 114% of polyoma genome length and, less frequently, of linked and multiple linked dimeric structures.