Abstract
Biofilm formation and dispersal in the black rot pathogen Xanthomonas campestris pathovar campestris (Xcc) is influenced by a number of factors. The extracellular mannanase ManA has been implicated in biofilm dispersal whereas biofilm formation requires a putative glycosyl transferase encoded by the xag gene cluster. Previously we demonstrated that the post-transcriptional regulator RsmA exerts a negative regulatory influence on biofilm formation in Xcc. Here we address the mechanisms by which RsmA exerts this action. We show that RsmA binds to the transcripts of three genes encoding GGDEF domain diguanylate cyclases to influence their expression. Accordingly, mutation of rsmA leads to an increase in cellular levels of cyclic di-GMP. This effect is associated with a down-regulation of transcription of manA, but an upregulation of xag gene transcription. Mutation of clp, which encodes a cyclic di-GMP-responsive transcriptional regulator of the CRP-FNR family, has similar divergent effects on the expression of manA and xag. Nevertheless Clp binding to manA and xag promoters is inhibited by cyclic di-GMP. The data support the contention that, in common with other CRP-FNR family members, Clp can act as both an activator and repressor of transcription of different genes to influence biofilm formation as a response to cyclic di-GMP.
Highlights
The formation, maturation and dispersal of biofilms are key processes within the lifestyle of many bacteria including animal and plant pathogens [1,2]
In order to address these issues, we examined the effect of rsmA mutation on the transcription of genes encoding proteins with GGDEF, EAL or HD-GYP domains by qRT-polymerase chain reaction (PCR) and assessed the physical binding of RsmA to transcripts by electrophoretic mobility shift assays (EMSA)
The findings support a model by which RsmA influences the expression of three genes encoding GGDEF domain proteins that leads to alteration in cellular levels of cyclic di-GMP and consequent divergent effects on the expression of manA and xag mediated by the Clp transcriptional regulator (Fig. 9)
Summary
The formation, maturation and dispersal of biofilms are key processes within the lifestyle of many bacteria including animal and plant pathogens [1,2]. The extracellular environment undoubtedly influences many aspects of bacterial behaviour including the dynamics of biofilm formation and dispersal. An array of signal transduction systems link the sensing of specific environmental cues to appropriate alterations in bacterial physiology and/or gene expression. An understanding of these different regulatory elements or pathways, how they are integrated and how they act to influence biofilm dynamics may have important implications for the control of bacterial disease
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