Changes in synthesis of DNA-binding proteins during the onset of transformation in NRK cells transformed by a temperature-sensitive mutant of Rous sarcoma virus.

Synthesis of cytoplasmic DNA-binding proteins was investigated after a shift from the nonpermissive to the permissive temperature in NRK cells transformed by a temperature-sensitive mutant of Rous sarcoma virus [ts339(RSV)]. Cells were labeled for several generations in [3H]leucine and were pulse-labeled with [35S]methionine for 1 h at the nonpermissive temperature (39 degrees C) and at the permissive temperature (33 degrees C, 5 h after shift from 39 degrees C). Proteins binding to sequential columns of double-stranded and single-stranded DNA-cellulose were examined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, and the 35S/3H ratios were obtained for each column fraction and for individual polypeptides. The protein fractions binding to single-stranded, but not double-stranded, DNA and eluting at high salt concentrations (greater than 0.60 M NaCl) showed elevated 35S/3H ratios. This indicated increased synthesis of these proteins within 5 h after the onset of transformation. The majority of the polypeptides in these fractions showed increased synthesis as a consequence of transformation. One prominent polypeptide among them constituted 0.1% of the cytosol protein and had a molecular weight of 93,000. We conclude that the synthesis of proteins binding tightly to single-stranded DNA is increased early after the onset of transformation.