TABLE 1.

Spontaneous mutation frequencies of various strains

Strain	Avg frequency of Rifr cells ± SDa	Relative mutation frequencyb
E. coli
BMH71-18 mutS (MutS−)	(3.5 ± 0.4) × 10−6	220
BMH71-18 mutS(pEEES)	(7.8 ± 1.5) × 10−8	1
BMH71-18 mutS(pEPPS)	(7.2 ± 1.3) × 10−7	10
B. subtilis
168trp (MutS+)	(8.7 ± 1.8) × 10−8	1
168trp-S (MutS−)	(2.9 ± 0.9) × 10−7	30
168trp-S(pBPPS)	(4.5 ± 1.6) × 10−8	2
P. putida
33015 (MutS+)	(1.3 ± 0.4) × 10−9	1
33015-S (MutS−)	(1.5 ± 0.5) × 10−6	1,000
33015-S(pPPPS)	(1.4 ± 0.5) × 10−9	1

^aAll strains grown to the stationary phase in Luria-Bertani medium were inoculated into this medium at approximately 50 cells/ml and grown for 25 generations. Aliquots of the cultures were then diluted and spread on Luria-Bertani plates containing rifampin for the selection of spontaneous mutants. Rifampin was used at the following concentrations: for E. coli, 100 μg/ml; for B. subtilis, 30 μg/ml; for P. putida, 50 μg/ml. Cell concentration was determined after the growth of 25 generations. To calculate the standard deviation, all of the experiments were repeated at least five times. Frequencies were calculated from both the total number of cells and the number of Rif^r cells. 

^bRelative frequencies were obtained by comparison with BMH71-18 mutS(pEEES) as 1 for E. coli, 168trp (MutS⁺) as 1 for B. subtilis, and 33015 (MutS⁺) as 1 for P. putida.