RsmA Regulates Biofilm Formation in Xanthomonas campestris through a Regulatory Network Involving Cyclic di-GMP and the Clp Transcription Factor

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.

Cyclic di-GMP signalling and the regulation of bacterial virulence

“…Several lines of evidence suggest that Clp plays a role in the regulation of biofilm dynamics in response to alterations in the cyclic di-GMP level. Mutation of clp leads to the downregulation of expression of manA that encodes the endomannanase ManA, which is implicated in biofilm dispersal and, conversely, in the upregulation of xag gene expression, which is implicated in biofilm formation (Lu et al , 2012). The binding of Clp to promoters of both manA and xag genes is inhibited by cyclic di-GMP.…”

“…The binding of Clp to promoters of both manA and xag genes is inhibited by cyclic di-GMP. A parsimonious explanation for these findings is that Clp can act both as an activator and a repressor of transcription of different genes (Lu et al , 2012). However, the evidence provided does not rule out the possibility that regulation of xag gene expression by Clp is more indirect, for example by positive regulation of a distinct repressor protein.…”

Small non-coding RNAs in plant-pathogenicXanthomonasspp

“…In both strains, deletion of rsmA led to a loss of pathogenicity, which was associated with an altered expression of hrp-and rpf-genes. [51][52][53] These mutants also showed pleiotropic phenotypes, e.g., higher levels of intracellular glycogen, increased bacterial adhesion and aggregation, and reduced production of extracellular enzymes. [51][52][53] RsmA in Xanthomonas spp.…”

Sustained sensing as an emerging principle in second messenger signaling systems

“…Conversely, processes that are known to be allosterically governed by c-di-GMP receptor proteins may also have transcriptional/translational c-di-GMP regulation. Examples of genes that encode biosynthetic operons that are regulated by c-di-GMP-binding transcription factors include the psl operon in P. aeruginosa via FleQ [4], the vps operon in V. cholerae via VpsT and VpsR [5,19,32], the xag operon in X. campestris via Clp [7,33], Bcam1330-Bcam1341 operon in Burkholderia cenocepacia via Bcam1349 [8] and E. coli curli [34–37]. Examples of processes that are regulated allosterically by c-di-GMP binding include cellulose synthesis via binding to BcsA [2,12,38,39] and BcsE in S. Typhimurium [40], alginate production via binding to Alg44 in P. aeruginosa [41,42] and cell adhesion via binding to both GcbC and LapD in P. fluorescens [43–45].…”