In P. putida KT2440, the cfaB gene is transcribed divergently with respect to the lpd3 gene encoding a dihydrolipoamide dehydrogenase and convergently with the cls (cardiolipin synthase) gene (Fig. 2a), suggesting that the cfaB gene is a monocistronic unit. In order to identify the promoter of the cfaB gene, we first determined the transcriptional start point (tsp) of the KT2440 cfaB by primer extension analysis. The tsp was found to be identical to that of the P. putida

DOT-T1E strain (Pini et al., 2009) and located 53 nucleotides upstream of the proposed ATG codon of the CFA sequence (Fig. 2b). Putative consensus sequences for the Shine–Dalgarno box and for the −35 and −10 boxes of an selleck chemicals RpoS-dependent promoter were found upstream from the transcription E7080 in vitro initiation point (Fig. 2b). To confirm that the expression from the cfaB promoter in this strain was RpoS-dependent, the cfaB promoter was fused to the ‘lacZ gene in plasmid pMP220 and β-galactosidase activity was measured in P. putida KT2440 and in its isogenic RpoS mutant (Ramos-González & Molin, 1998). As can be seen in Fig. 2c, expression of the cfaB promoter in

P. putida KT2440 was fully dependent on the growth phase and no expression was detected in the RpoS knockout mutant strain. As expected, real-time PCR assays showed that the expression of rpoS and cfaB was almost nonexistent in the exponential growth phase, while both genes were expressed at a relatively high level during the stationary phase (Fig. 2d). cfaB expression started to decrease slightly before the expression of the rpoS gene. In the cfaB promoter, the proposed consensus sequence for RpoS recognition differs only in one position from the E. coli consensus (Fig. 3a) and it covers mafosfamide from the bases from −8 to −14 rather than −7 to −13. To analyze the importance of each nucleotide in the putative RpoS recognition site of the cfaB promoter, we generated transverse

point mutations in each of the seven nucleotides between positions −8 and −14 (Fig. 3b). The mutant promoters were cloned into the pMP220 plasmid and β-galactosidase expression was followed throughout the growth curve. Expression from wild-type and mutant promoters during the exponential phase of growth was low (never higher than 100 Miller Units) (not shown). However, the expression increased when the culture reached a turbidity at 660 nm of approximately 1.5 and high levels (1300 Miller Units) were detected when the cultures had reached a turbidity of 3 (Fig. 3b). Mutations at positions −14, −13, −12 and −9 completely abolished the cfaB promoter activity.