Different Chinese hamster cell lines express a G1 period for different reasons

Previous studies from our laboratory have shown that the absence of G1 (G1^- condition) in two lines of Chinese hamster cells is dominant over the presence of G1 (G1⁺ condition) in a variety of intraspecific cell hybrids. G1⁺ mutants or variants can be isolated from G1^- cells following mutagenesis and selection. These G1⁺ mutants fall into multiple complementation groups based on their abilities to form G1^- cell hybrids with one another. This is evidence that different mutants have G1 intervals for different reasons, possibly as the result of deficiencies in functions necessary for G1^- cell cycles. In this report we have used cell hybrid analysis to ask whether cells of different, naturally occurring G1⁺ lines of Chinese hamster are able to complement to produce G1^- hybrids. We have found three complementation groups among the four G1⁺ cell lines examined. Therefore, these lines define three different reasons or bases for the existence of a G1 interval. These results lead us to suggest that multiple requirements must be met for these cells to start the S period, but that failure to fulfill only a single and different requirement is responsible for the presence of a G1 interval in any given cell line.