Genetic analysis of the G1 period: Isolation of mutants (or variants) with a G1 period from a Chinese hamster cell line lacking G1

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Abstract
Cells of the Chinese hamster line V79-8 multiply without a G1 period (i.e., they are G1-) and have an average generation time of 9.5 h. After mutagenesis and selection 5 stable mutants (or variants) of this line were derived that have longer generation times. In each case the increase in generation time is due solely to the introduction of a G1 period into the cell cycle, with no measurable effect on S, G2 or M. Fusions among these 5 G1+ mutant lines and another presumably nonmutant G1+ line (V79-743) produce hybrid cells lacking a G1 period in all but 1 case. These complementation tests define 5 complementation groups among these 6 G1+ cell lines. The 6 G1+ lines represent 5 different causes or bases for the presence of a G1 period. The 2 G1+ mutants belonging to complementation group V are temperature sensitive for expression of the G1+ phenotype (G1 .simeq. 0, 4 and 6 h at 33.degree., 37.degree. and 39.degree. C, respectively). In all cases the G1- state is dominant over the G1+ state, suggesting that the presence of G1 represents a deficient condition. Mutants of this type may be useful in the analysis of the switch from G1- to G1+ that occurs normally in cleaving embryos and in elucidation of the genetic mechanism(s) responsible for the presence of a measurable G1 in most cells.