Transcription activation of two clusters for exopolysaccharide biosynthesis by phosphorylated DctD in Vibrio vulnificus

Abstract

NtrC-mediated production of exopolysaccharides (EPS), essential components for Vibrio vulnificus biofilms, is highly increased in the presence of dicarboxylic or tricarboxylic acids. Gel-shift assays showed that regulation of the EPS-gene cluster I (EPS-I cluster) by NtrC was direct via binding of phosphorylated NtrC (p-NtrC) to the regulatory region of the EPS-I cluster. In contrast, p-NtrC did not bind to the EPS-II and EPS-III clusters, suggesting that NtrC regulation was not direct and another transcription factor belonging to an NtrC-regulon might play a role in activating their transcription. A candidate transcription factor, DctD, of which expression was induced by NtrC, activated the expression of the EPS-II and EPS-III clusters via direct binding to their upstream regions. Under growth conditions with either dicarboxylic or tricarboxylic acids, the expression of NtrC was induced and the transcription of dctD was activated. Furthermore, DctD exhibited higher transcriptional activity under the conditions with dicarboxylic acids than with tricarboxylic acids. Therefore, this study demonstrates that under dicarboxylate-rich conditions, both the abundance and activity of DctD were markedly induced, which activates the expression of two EPS clusters to maximize biosynthesis of EPS facilitating biofilm maturation in V. vulnificus.

Transition of Dephospho-DctD to the Transcriptionally Active State via Interaction with Dephospho-IIA Glc

“…Its sensor kinase, DctB, senses the ambient dicarboxylic acids and phosphorylates DctD, which in turn activates the transcription of genes encoding the uptake system(s) for dicarboxylic acids ( 15 ). In addition to nitrogen-fixing bacteria, many bacterial species, including the model foodborne pathogen Vibrio vulnificus , are also equipped with DctBD ( 16 ). In the genomes of V. vulnificus , two operons encoding the DctBD homologs, dctB 1 D 1 and dctB 2 D 2 , are present, but it was found that DctD 2 is responsible for activating the transcription of exopolysaccharide (EPS) EPS-II and EPS-III clusters in the presence of fumarate ( 16 ).…”

“…In addition to nitrogen-fixing bacteria, many bacterial species, including the model foodborne pathogen Vibrio vulnificus , are also equipped with DctBD ( 16 ). In the genomes of V. vulnificus , two operons encoding the DctBD homologs, dctB 1 D 1 and dctB 2 D 2 , are present, but it was found that DctD 2 is responsible for activating the transcription of exopolysaccharide (EPS) EPS-II and EPS-III clusters in the presence of fumarate ( 16 ). Notably, biosynthesis of EPSs, the loosely associated polysaccharides outside bacterial envelopes ( 17 ), is regulated at the transcription level via the involvement of DctD 2 , which activates the expression of EPS gene clusters in the presence of various kinds of dicarboxylic acids ( 16 ).…”

“…In the genomes of V. vulnificus , two operons encoding the DctBD homologs, dctB 1 D 1 and dctB 2 D 2 , are present, but it was found that DctD 2 is responsible for activating the transcription of exopolysaccharide (EPS) EPS-II and EPS-III clusters in the presence of fumarate ( 16 ). Notably, biosynthesis of EPSs, the loosely associated polysaccharides outside bacterial envelopes ( 17 ), is regulated at the transcription level via the involvement of DctD 2 , which activates the expression of EPS gene clusters in the presence of various kinds of dicarboxylic acids ( 16 ). Furthermore, the transcription of the dctB 2 D 2 operon is activated by NtrB/C, the TCS responsive to the carbon/nitrogen ratio ( 16 ).…”

“…Notably, biosynthesis of EPSs, the loosely associated polysaccharides outside bacterial envelopes ( 17 ), is regulated at the transcription level via the involvement of DctD 2 , which activates the expression of EPS gene clusters in the presence of various kinds of dicarboxylic acids ( 16 ). Furthermore, the transcription of the dctB 2 D 2 operon is activated by NtrB/C, the TCS responsive to the carbon/nitrogen ratio ( 16 ). The findings that the production and secretion of EPSs, which are presumed to be entirely composed of carbohydrates, are regulated by both NtrB/C and DctB 2 /D 2 have implied the presence of a common regulatory connection or connections in bacterial recognition of the depletion of nitrogen sources and the repletion of some carbon sources.…”

“…The EPS-II and EPS-III clusters are required for production of EPSs involved in biofilm maturation under the conditions containing dicarboxylic acids, although EPS compositions have not yet been elucidated in this bacterial species ( 28 , 31 ). One of the major regulators for induction of these clusters has been shown to be DctB/D ( 16 ).…”