The platelet-derived growth factor (PDGF) stimulates density-arrested BALB/c-3T3 cells to synthesize a protein (pII; Mr, 35,000) that is constitutively synthesized by spontaneously transformed BALB/c-3T3 (ST2-3T3) cells which do not require PDGF for growth. Antisera against a major excreted protein family (MEP) of retrovirus-transformed cells quantitatively precipitated cellular pII. PDGF-stimulated pII has the same molecular weight, a similar charge, and similar antigenic determinants as authentic MEP isolated from ST2-3T3 or retrovirustransformed cells. MEP represented about 2% of the nonnuclear proteins synthesized by ST2-3T3 cells and 0.3 to 0.6% of the proteins synthesized by PDGFtreated BALB/c-3T3 cells, a three-to sixfold increase over the background. In BALB/c-3T3 cells, less PDGF was required for pII (MEP) synthesis than for DNA synthesis. PDGF induced a selective increase in pII (MEP) within 40 min. Such preferential synthesis was inhibited by brief treatment with actinomycin D, suggesting a requirement for newly formed RNA. The constitutive synthesis of pll (MEP) by ST2-3T3 cells was not inhibited by actinomycin D. Five spontaneously or chemical carcinogen-transformed tumorigenic BALB/c-3T3 cell lines were studied; they neither required PDGF for growth nor responded to it. These cell lines became arrested at confluence with a G1 DNA content. Each of these independently isolated lines synthesized pII (MEP) constitutively. Thus, the synthesis of pII (MEP) may be required, but is not sufficient, for PDGFmodulated DNA synthesis.

The in vitro survival of Escherichia coli O157:H7 at 15°C under two experimental conditions (sterile soil and sterile natural water) was examined. DNA microarrays of the entire set of E. coli O157:H7 genes were used to measure the genomic expression patterns after 14 days. Although the populations declined, some E. coli O157:H7 cells survived in sterile stream water up to 234 days and in sterile soil for up to 179 days. Cells incubated in soil microcosms for 14 days expressed genes for antibiotic resistance, biosynthesis, DNA replication and modification, metabolism, phages, transposons, plasmids, pathogenesis and virulence, antibiotic resistance, ribosomal proteins, the stress response, transcription, translation, and transport and binding proteins at significantly higher levels than cells grown in Luria broth. These results suggest that E. coli O157:H7 may develop a different phenotype during transport through the environment. Furthermore, this pathogen may become more resistant to antibiotics making subsequent infections more difficult to treat.

Quiescent cultures of density arrested BALB/c-3T3 cells have been sensitized to the growth stimulatory action of the platelet-derived growth factor (PDGF). Sensitization was achieved by depriving the cultures of PDGF prior to growth stimulation and was noted after transfer of cultures from medium supplemented with 10% serum to medium containing either an equivalent concentration of platelet-poor plasma or a low concentration (0.5%) of serum. Sensitized cultures required less pure PDGF for growth stimulation than nonsensitized ones. In addition such cultures required less mitogen to synthesize a PDGF modulated major excreted protein (MEP). The mechanism of sensitization was investigated. Sensitized cultures did not bind more PDGF than non-sensitized ones. Rather, sensitization appeared to result from the loss of cells that occurred when cultures were deprived of PDGF. Such a loss increased the amount of PDGF available per cell, causing a higher percentage of cells to enter the S phase. Similarly, the amount of PDGF per cell regulated MEP synthesis. Furthermore, in non-sensitized cultures (containing the same number of cells), the absolute quantity rather than the concentration of PDGF regulated DNA synthesis. It appears that the amount of PDGF per cell modulates mitogenesis.

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