Temperature-sensitive mutants of a Chinese hamster cell line. I. Selection of clones with defective macromolecular biosynthesis.

Temperature-sensitive clones have been selected from a mutagenized culture of Chinese hamster lung cells by a procedure involving bromodeoxyuridine (BrdU) incorporation and irradiation with black light. The selection procedure used in these studies was adapted from methods developed by others to yield mutants that cease DNA replication within a short time after they are transferred to nonpermissive temperature. After mutagenesis with ethyl methanosulfonate ten clones survived the selection procedure. Three of the clones (mutants) were temperature-sensitive as measured by growth properties. Two mutants ceased DNA synthesis within six hours of being shifted to 39degrees and the third mutant continued to synthesize DNA at nonpermissive temperature at a reduced rate for at least 24 hours. Thus, all three mutants survived the selection procedure for understandable reasons, since each was unable to incorporate sufficient BrdU at 39degrees to lethally protosensitize its DNA during the standard exposure period. The two mutants that cease DNA synthesis at high temperature (clones 115-47 and 115-53) also stop incorporating radioactive amino acids and uridine within six hours at 39degrees. Their complex phenotype, i.e. defective DNA, RNA and protein biosynthesis, is reversible. When these mutants were returned to 33 degrees after 8 hours at 39 degrees, both resumed DNA synthesis immediately (less than 1 hour). Reversal of defective DNA synthesis in both mutants were sensitive to drugs that inhibit protein biosynthesis specifically. Those same drugs, as well as toxic amino acids analogs, also effected a striking mutant phenocopy in wild-type cells. The phenocopy produced by amino acid analogs that are incorporated into mammalian proteins suggested that one or more proteins must be synthesized continuously to support mammalian cells engaged in programmed DNA replication.