5 min, and an srl∷Tn10 marker at 60.9 min, gave rise to TetR NalR recombinants that grew to 109 CFU mL−1 before entering the stationary phase. We found that all of the recombinants

with the wild-type growth phenotype were also Thy+, presumably having received the thyA+ allele from the donor strain. (The thyA gene is located at 63.8 min.) At the same time, we discovered that the isolate of YK4131 we had received did not have the thyA mutation listed in its genotype, but was apparently a Thy+ revertant. These findings prompted us to check whether the difference in the Thy phenotype was responsible for the growth difference between YK410 and YK4131. We found that YK410 grew to 1.2 × 109 CFU mL−1 when the medium was supplemented with additional thymidine (200 μg mL−1), while the growth of YK4131 was unaffected. selleck Four independent spontaneous Thy+ revertants of YK410 were isolated and shown to grow to 1.3±0.2

× 109 CFU mL−1 before selleck chemicals llc entering the stationary phase, while in the same experiment, YK410 grew to only 2.7±0.2 × 108 CFU mL−1. Identical results were obtained when a thyA+ allele was introduced into YK410 by transduction selecting for a linked marker (data not shown). In parallel experiments, a thyA∷Tn10 mutation was introduced into YK4131. The YK4131 thyA∷Tn10 transductants grew to only 1.0±0.4 × 108 CFU mL−1 before entering the stationary phase, which was approximately 10% of the final growth yield of YK4131, which grew to 1.2±0.0 × 109 CFU mL−1. The thyA∷Tn10 mutation was also introduced into strains MG1655 and RP437, with comparable results. Cultures P-type ATPase of these thyA∷Tn10 transductants entered the stationary phase when the number of CFU mL−1 was only 20±1.0% of the number present in the thyA+ parent. We started these studies on flhD because of our interest in the stationary-phase-induced mcb operon promoter (Hernández-Chico et al., 1986; Connell et al., 1987). It had been reported that the level of the stationary-phase

expression of a Pmcb-lacZ reporter in YK4131 was only 10% of the level seen in YK410 (Connell, 1989). We had previously isolated deletion and point mutations in the mcb operon promoter (Pmcb) that identified promoter regions required for full promoter activity and stationary-phase regulation (Mao & Siegele, 1998). To determine whether of any of these promoter mutations altered interactions with FlhD, we first introduced a Pmcb-lacZ operon fusion into YK410 and YK4131 by lysogenization with λWM7 (Mao & Siegele, 1998). The flhD∷Tn10 mutation was transduced from YK4159 into YK410 (λPmcb-lacZ) to produce strain DS478 [YK410 (λPmcb-lacZ) flhD∷Tn10]. Pmcb promoter activity was assayed by measuring β-galactosidase levels throughout growth (Table 2). In the stationary phase, YK4131 (λPmcb-lacZ) had only 20% of the level of β-galactosidase activity as the flhD+ parental strain.