TABLE 2.

Appearance of tryptophan-independent mutants of CM1339 (mutT) and CM1377 (mutT polA) over 5 days on glucose minimal plates reflecting DNA synthesis^a

Strain	Mean mutants/plate on day:		Mean Y − X	Mean viable bacteria/plate on day 2 ± SE (106)	Mean Y − X/viable cell ± SE (10−6) [no. of expts]
	2 (X)	5 (Y)
CM1339 (set A)	19.6	59.9	40.3	5.78 ± 1.34	6.82 ± 1.22 [4]
CM1339 (set B)	13.9	24.1	9.7	8.34 ± 2.56	1.37 ± 0.48 [6]
CM1377 (set B)	12.0	17.9	5.34	6.75 ± 2.3	0.91 ± 0.38 [4]

^aThe mean initial plating density was close to 10⁷ plus 10⁸ scavenger bacteria of the kanamycin-sensitive strain WP2. Colonies existing on day 2 were those existing in the culture at the time of plating. In set B, these were removed with a cork borer. The number of viable cells of CM1339 or CM1377 was determined for a parallel set of plates by removing any mutant colonies, washing the bacteria off, and plating them on kanamycin plates. In some experiments, one or two mutants were seen on control plates with scavenger bacteria only. These were subtracted from the experimental plate numbers to give the values presented. The means in columns 2 to 5 are of individual experiments carried out on different days with different batches of plates. To obtain meaningful standard errors for the mutation rate, the values in column 6 are the means of the ratios for the individual experiments and not the ratios of the means which are marginally different.