Xanthomonas oryzae pv. oryzae recA is transcribed and regulated from multiple promoters

Cefoperazone induces esterase B expression by EstR and esterase B enhances cefoperazone activity at the periplasm

The occurrence of antibiotic resistance bacteria has become a major threat to public health. We have recently discovered a transcriptional activator that belongs to MarR family, EstR, and an esterase B (EstB) with a newly proposed de-arenethiolase activity from Sphingobium sp. SM42. De-arenethiolase activity involves the removal of the small aromatic side chain of cephalosporin antibiotics as an excellent leaving group by the enzymatic CS bond cleavage. Here, we report the regulation of estB through EstR as an activator in response to a third generation cephalosporin, cefoperazone, antibiotic. Cefoperazone induced the expression of estB in wild type Sphingobium sp., but not in the estR knockout strain, and the induction was restored in the complemented strain. Moreover, we revealed the importance of EstB localization in periplasm. Since EsB has the ability to inactivate selected β-lactam antibiotics in vitro, it is possible that the enzyme works at the periplasmic space of Gram negative bacteria similar to β-lactamases. EstB was genetically engineered by incorporating NlpA binding motif, or OmpA signal sequence, or SpyTag-SpyCatcher to the estB gene to mobilize it to different compartments of periplasm; inner membrane, outer membrane, and periplasmic space, respectively. Surprisingly, we found that Sphingobium sp. SM42 and E. coli expressing EstB at the periplasm were more sensitive to cefoperazone. The possible drug enhancement mechanism by enzyme was proposed. This work might lead to a novel strategy to tackle antibiotic resistance problem.

Characterization of the two Mycobacterium tuberculosis recA promoters

The recA gene of Mycobacterium tuberculosis is unusual in that it is expressed from two promoters, one of which, P1, is DNA damage inducible independently of LexA and RecA, while the other, P2, is regulated by LexA in the classical way (E. O. Davis, B. Springer, K. K. Gopaul, K. G. Papavinasasundaram, P. Sander, and E. C. Böttger, Mol. Microbiol. 46:791-800, 2002). In this study we characterized these two promoters in more detail. Firstly, we localized the promoter elements for each of the promoters, and in so doing we identified a mutation in each promoter which eliminates promoter activity. Interestingly, a motif with similarity to Escherichia coli sigma(70) -35 elements but located much closer to the -10 element is important for optimal expression of P1, whereas the sequence at the -35 location is not. Secondly, we found that the sequences flanking the promoters can have a profound effect on the expression level directed by each of the promoters. Finally, we examined the contribution of each of the promoters to recA expression and compared their kinetics of induction following DNA damage.

DNA damage induction of recA in Mycobacterium tuberculosis independently of RecA and LexA

The ubiquitous and highly conserved RecA protein is generally expressed from a single promoter, which is regulated by LexA in conjunction with RecA. We show here using transcriptional fusions to a reporter gene that the Mycobacterium tuberculosis recA gene is expressed from two promoters. Although one promoter is clearly regulated in the classical way, the other remains DNA damage inducible in the absence of RecA or when LexA binding is prevented. These observations demonstrate convincingly for the first time that there is a novel mechanism of DNA damage induction in M. tuberculosis that is independent of LexA and RecA.

Genetic organization of the lexA, recA and recX genes in Xanthomonas campestris

A gene cluster containing lexA, recA and recX genes was previously identified and characterized in Xanthomonas campestris pathovar citri (X. c. pv. citri). We have now cloned and sequenced the corresponding regions in the Xanthomonas campestris pv. campestris (X. c. pv. campestris) and Xanthomonas oryzae pathovar oryzae (X. o. pv. oryzae) chromosome. Sequence analysis of these gene clusters showed significant homology to the previously reported lexA, recA and recX genes. The genetic linkage and the deduced amino acid sequences of these genes displayed very high identity in different pathovars of X. campestris as well as in X. oryzae. Immunoblot analysis revealed that the over-expressed LexA protein of X. c. pv. citri functioned as a repressor of recA expression in X. c. pv. campestris, indicating that the recombinant X. c. pv. citri LexA protein was functional in a different X. campestris pathovar. The abundance of RecA protein was markedly increased upon exposure of X. c. pv. campestris to mitomycin C, and an upstream region of this gene was shown to confer sensitivity to positive regulation by mitomycin C on a luciferase reporter gene construct. A symmetrical sequence of TTAGTAGTAATACTACTAA present within all three Xanthomonas lexA promoters and a highly conserved sequence of TTAGCCCCATACCGAA present in the three regulatory regions of recA indicate that the SOS box of Xanthomonas strains might differ from that of Escherichia coli.

Characterization of Xanthomonas oryzae pv. oryzae recX, a gene that is required for high-level expression of recA

Analysis of the nucleotide sequence downstream from the Xanthomonas oryzae pv. oryzae recA gene reveals two orfs designated orfX and recX. The former has the potential to code for a 5.6 kDa protein of unknown function while the latter encodes for a putative 14.6 kDa protein with homology to RecX from various bacteria. Northern blot analysis and RT-PCR results show that recA-orfX-recX are co-regulated and arranged in an operon. A recX mutant was constructed. The mutant has no obvious growth defects or stress response defects, except that it cannot support high-level expression of recA from an expression vector. Introduction of the plasmid containing recA into the recX mutant resulted in reduced transformation efficiency and all transformants tested had mutations with reduced RecA levels. Moreover, the recX mutant has reduced basal levels of RecA. This has not been observed in other bacteria. When inactivated recX was complemented in trans, both changes were reversed. recX mutation has no effect on the regulation of the recA promoter, suggesting that its effect on the RecA level could be post-transcriptional.

Complex regulation of the organic hydroperoxide resistance gene (ohr) from Xanthomonas involves OhrR, a novel organic peroxide-inducible negative regulator, and posttranscriptional modifications

Analysis of the sequence immediate upstream of ohr revealed an open reading frame, designated ohrR, with the potential to encode a 17-kDa peptide with moderate amino acid sequence homology to the MarR family of negative regulators of gene expression. ohrR was transcribed as bicistronic mRNA with ohr, while ohr mRNA was found to be 95% monocistronic and 5% bicistronic with ohrR. Expression of both genes was induced by tert-butyl hydroperoxide (tBOOH) treatment. High-level expression of ohrR negatively regulated ohr expression. This repression could be overcome by tBOOH treatment. In vivo promoter analysis showed that the ohrR promoter (P1) has organic peroxide-inducible, strong activity, while the ohr promoter (P2) has constitutive, weak activity. Only P1 is autoregulated by OhrR. ohr primer extension results revealed three major primer extension products corresponding to the 5' ends of ohr mRNA, and their levels were strongly induced by tBOOH treatment. Sequence analysis of regions upstream of these sites showed no typical Xanthomonas promoter. Instead, the regions can form a stem-loop secondary structure with the 5' ends of ohr mRNA located in the loop section. The secondary structure resembles the structure recognized and processed by RNase III enzyme. These findings suggest that the P1 promoter is responsible for tBOOH-induced expression of the ohrR-ohr operon. The bicistronic mRNA is then processed by RNase III-like enzymes to give high levels of ohr mRNA, while ohrR mRNA is rapidly degraded.