Figure 3 shows that there was a gradual decrease in the ThyA level during the stationary growth phase to 40% of that in the LDK378 in vitro late-exponential phase cells in LB medium (Fig. 3a and c). Conversely, ThyX was maintained at the same

level in both the late-exponential and stationary phase cells (Fig. 3b and c), indicating that the levels of ThyA and ThyX were regulated by different mechanisms and that ThyX could play a role in the stationary growth phase of C. glutamicum. The thyX gene is located on an operon with dapB and dapA, and these genes are transcribed as a single unit, dapB-thyX-dapA (Park et al., 2010). Two putative promoter regions of dapB were identified by primer extension analyses (Pátek et al., 1996), and one of the promoters or both (p1-dapB and/or p2-dapB) might be recognized by SigB. SigB was shown to be induced during the transition from the exponential to the stationary growth phase (Larisch et al., 2007; Pátek & Nešvera, 2011).

To examine whether the level of ThyX was regulated by SigB, a ΔsigB strain was constructed by allelic replacement using a sucrose counter-selectable suicide plasmid. Deletion of sigB was confirmed Z-VAD-FMK nmr by PCR amplification of the sigB region, with primers binding upstream and downstream of sigB. A 1329-bp fragment containing intact sigB was seen in the wild-type strain, and a 324-bp fragment was seen in the mutant strain (Fig. 1b). The transcriptional activity of the dapB-thyX promoter region was quantified in the wild-type and ΔsigB strain KH4 after the

introduction of plasmid pMTXL1. The thyX promoter in the ΔsigB strain revealed about 25% of the activity shown in the parental wild-type strain (Fig. 4a). Thus, SigB was shown to be necessary for the induction of thyX. The levels of ThyA or ThyX in the wild-type, KH4, and KH5 strains of C. glutamicum were analyzed by immunoblotting using antiserum against ThyA or ThyX, respectively. Whereas the level of ThyA in the ΔsigB strain was comparable to that of the parental wild-type, the level of ThyX was diminished significantly in the deletion mutant (Fig. 4b). Complementation of the ΔsigB mutation was performed with a plasmid containing wild-type sigB, including its putative promoter region. Western blotting analysis revealed that expression Rho of functional sigB in the complemented strain restored the accumulation of ThyX to nearly wild-type levels (Fig. 4b). This result confirmed that SigB is necessary for maintenance of the level of ThyX during transition into the stationary growth phase. To investigate the role of the sigma factor SigB on sensitivity to a DHFR inhibitor, WR99210-HCl, wild-type, KH4, and KH5 strains grown to log-phase were inoculated into MCGC minimal medium containing isocitrate and glucose with 3 µM WR99210-HCl. Growth was monitored for 36 h, and the KH4 strain appeared to be sensitive to WR99210-HCl.