We used the P. aeruginosa PAO1 strain containing pAB134, which

carries the compound screening assay LuxCDABE operon under the control of the rhlG promoter region (prrhlG), extending from − 413 to −23 relative to the first base of the rhlG translation initiation codon. We chose this strain since the multi-copy pAB134 plasmid led to higher amounts of mRNAs than the genomic mono-copy rhlG gene, thereby facilitating the experiment. Three internal luxCDABE primers CA3 were used to synthesize cDNAs and amplify them by PCR. A mix of two DNA fragments, both of ~ 400 pb was obtained after the last PCR. They were sequenced, identifying two different transcription start sites at positions −113 and −55 relative to the rhlG translation initiation codon (Figure 1). The weakest signal (−55) corresponded to the transcription start site previously identified by Campos Garcia et al. [4] as arising from a σ70-dependent promoter. The strongest signal (−113) revealed a novel transcription start site preceded by the sequence CAACCT − N16 − TCTG,

Selleckchem CX5461 which is similar to the consensus sequence for AlgU-dependent promoters, GAACTT − N16–17 − TCTG [20]. AlgU is the extra-cytoplasmic function (ECF) sigma factor involved in alginate overproduction leading to mucoidy, response to some stresses, and biofilm stability [21–23]. Figure 1 Promoter mapping of rhlG. A: Schematic representation of the rhlG locus. Black flags indicate the promoters PAlgU, Pσ54, and Pσ70; and arrows indicate the rhlG and PA3388 genes. B: Annotated sequence of the rhlG promoter region. Black triangles indicate the three transcription start sites (+1) and the negative numbers provide their position relative to the rhlG translation initiation codon. The promoter sequences recognized by the sigma factors AlgU, σ54, and σ70 are respectively point over lined, full trait over lined, and underlined. The “lux box” as proposed in [4] is boxed with the two highly conserved dinucleotides Ribonucleotide reductase underlined. The

chromatograms show the results of 5′-RACE PCR allowing us to identify the major transcription start sites resulting from PAlgU and the minor from 1 Pσ70, the white arrow corresponding to the last base before the polyC tail added to the 5′ extremity of cDNA. The transcription start site resulting from Pσ54 was identified in [4]. The pAB134 plasmid was primarily constructed to quantify the prrhlG activity in the course of bacterial growth by measuring the luminescence resulting from the LuxCDABE proteins. To verify the role of AlgU in the transcription of rhlG, P. aeruginosa PAO1 and its algU mutant strain PAOU [21] were transformed by pAB133 (containing the promoter-less luxCDABE operon, used to quantify the luminescence baseline) and pAB134. Strains were grown in PPGAS medium and luminescence was followed during 30 h.