The effect of heat treatment at 42 °C on a thermosensitive division-defective strain of K12, maci, has been studied under conditions which support a generation time of about 50 min. Synchronous cells gained simultaneously the ability to divide at 42 °C and to divide in the presence of nalidixic acid or chloramphenicol, 20 min before physical separation of daughter cells. When synchronous cells of different ages (between o and 20 min after elution from an absorbent membrane) were subjected to a heat shock, division always took place 55 to 60 min after the shock. A similar treatment of an exponential culture resulted in synchronous cell division after a lag of 55 to 60 min during which no division ocfnred. Division is probably controlled for 40 to 45 min by the gene mutated in maci. Thus maci cells of different ages appear to return to the same point of their division cycle when they are heated at 42 °C. We propose that the gene mutated in maci has a role in the timing control of cell division. Progress to division appears to require a fixed period in which the function controlled by the gene is performed; this period ends, under physiological conditions, when division does not require further protein or DNA synthesis.


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