Protein “X” Synthesis in Mammalian Cells

The unimpeded progress of proliferation and replication should be monitored precisely by each cell. In E. coli, such a monitoring system has indeed been detected. Upon inhibition of proliferation, it generates signals which cause the induction of “protein X” (= recA product) (Inouye and Pardee, 1970; Gudas and Pardee, 1976; McEntee, 1977). This protein then elicits the appearance of “SOS functions” (for reviews see Witkin, 1976; and Devoret, 1978). It seems that some of these SOS functions may also exist in eukaryotic cells, e.g. Weigle reactivation (Sarasin and Hanawalt, 1978; Lytle and Goddard, 1979) and pro-virus induction (Links et al., 1977). We examine here whether mammalian cells respond to an inhibition of replication by induction of new proteins*.