It has been long considered that zinc homeostasis in bacteria is maintained by export systems and uptake systems, which are separately controlled by their own regulators and the uptake systems are negatively regulated by Zur which binds to an about 30-bp AT-rich sequence known as Zur-box present in its target promoters to block the entry of RNA polymerase. Here, we demonstrated in vivo and in vitro that in addition to act as a repressor of putative Zn2+-uptake systems, the Zur of the bacterial phytopathogen Xanthomonas campestris pathovar campestris (Xcc) acts as an activator of a Zn2+ efflux pump. The Xcc Zur binds to a similar Zur-box with ∼30-bp AT-rich sequence in the promoters of the genes encoding putative Zn2+-uptake systems but a 59-bp GC-rich sequence with a 20-bp inverted repeat overlapping the promoter's −35 to −10 sequence of the gene encoding a Zn2+-export system. Mutagenesis of the inverted repeat sequence resulted in abolishment of the in vitro binding and the in vivo and in vitro activation of the export gene's promoter by Zur. These results reveal that the Xcc Zur functions as a repressor and an activator of putative zinc homeostasis genes via recognizing two distinct sequences within its target promoters.

The alk genes of Pseudomonas oleovorans, which is able to metabolize alkanes and alkenes, are organized in alkST and alkBFGHJKL clusters, in which the expression of alkBFGHJKL is positively regulated by AlkS. Growth of the wild-type strain GPo1 and P. oleovorans GPo12 alk recombinants on octane resulted in changes of cellular physiology and morphology. These changes, which included lower growth rates and a reduction of the number of CFU due to filamentation, were also seen when the cells were grown on aqueous medium, and the alk genes were induced with dicyclopropylketone, a gratuitous inducer of the alk genes. These effects were seen only for recombinants carrying both alkST and alkBFGHJKL operons. Deletion of parts of either alkB or alkJ, which encode two major Alk proteins located in the cytoplasmic membrane, modified but did not eliminate the effects described above, suggesting that they were due to induction and expression of several alk genes. Continuous growth of the cells in the presence of dicyclopropylketone for about 10 generations led to inactivation, but not elimination, of the alk genes. This resulted in a return of the recombinants to normal physiology and growth.Pseudomonas oleovorans is able to grow on alkanes and alkenes (1). Its catabolic OCT plasmid contains two operons that encode the genes necessary for alkane oxidation (Fig. 1). Both operons have been sequenced (11,17,18,27,29), and it has been shown that the expression of operon alkB-L is positively regulated by AlkS (10,29). The alkane hydroxylase system, which catalyzes the first step of alkane oxidation (23), consists of three components: alkane hydroxylase (AlkB), rubredoxin (AlkG), and rubredoxin reductase (AlkT) (11,17,18,26). The enzymes which catalyze the subsequent oxidation and catabolic steps are encoded by alkJ, alkH, and alkK (17,27). alkF encodes a nonfunctional rubredoxin (17), while alkL encodes an outer membrane protein which is not essential for growth on octane (27). When the alk system is induced with octane or the gratuitous inducer dicyclopropylketone (DCPK), about 35,000 copies of AlkB (12) and 10,000 copies of AlkJ (27) per cell are found in the cytoplasmic membrane. The former represents 25 to 30%, while the latter represents about 10% of total cytoplasmic membrane proteins.When P. oleovorans is grown on alkanes or alkenes, the cells change their physiology, membrane morphology (8, 24), and membrane properties (7). Changes of membrane lipid fatty acid composition resulted both from exposure of the cells to organic solvents and from the induction and expression of alkB (6). Induction of alkB and insertion of alkane hydroxylase into the cytoplasmic membrane have also been found to alter the physiology of Escherichia coli alk ϩ recombinants, by inducing, for example, reductions in growth rate and appearance of membrane vesicles in the cytoplasm (15,22).To explore the effects of induction and expression of the alk genes, and especially alkB, we have compared the physiological and morphological changes of Pseudomonas h...

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The RNA chaperone, Hfq, is known to play extensive roles in bacterial growth and development. More recently, it has been shown to be required for virulence in many human and animal bacterial pathogens. Despite these studies little is known about the role Hfq plays in phytopathogenic bacteria. In this study, we show Hfq is required for full virulence of the crucifer black rot pathogen Xanthomonas campestris pv. campestris (Xcc). We demonstrate that an Xcc hfq deletion strain is highly attenuated for virulence in Chinese radish and shows a severe defect in the production of virulence factors including extracellular enzymes and extracellular polysaccharide. Furthermore, the Xcc strain lacking Hfq had significantly reduced cell motility and stress tolerance. These findings suggest that Hfq is a key regulator of important aspects of virulence and adaptation of Xcc. Taken together, our findings are suggestive of a regulatory network placing Hfq at the centre of virulence gene expression control in Xcc.

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