| Summary: | Vibrios can degrade chitin surfaces to soluble N-acetyl glucosamine oligosaccharides (GlcNAc<sub>n</sub>) that can be utilized as a carbon source and also induce a state of natural genetic competence. In this study, we characterized chitin-dependent growth and natural competence in <i>Vibrio parahaemolyticus</i> and its regulation. We found that growth on chitin was regulated through chitin sensors ChiS (sensor histidine kinase) and TfoS (transmembrane transcriptional regulator) by predominantly controlling the expression of chitinase VPA0055 (ChiA2) in a TfoX-dependent manner. The reduced growth of Δ<i>chiA2</i>, Δ<i>chiS</i> and Δ<i>tfoS</i> mutants highlighted the critical role played by ChiA2 in chitin breakdown. This growth defect of Δ<i>chiA2</i> mutant could be recovered when chitin oligosaccharides GlcNAc<sub>2</sub> or GlcNAc<sub>6</sub> were supplied instead of chitin. The Δ<i>tfoS</i> mutant was also able to grow on GlcNAc<sub>2</sub> but the Δ<i>chiS</i> mutant could not, which indicates that GlcNAc<sub>2</sub> catabolic operon is dependent on ChiS and independent of TfoS. However, the Δ<i>tfoS</i> mutant was unable to utilize GlcNAc<sub>6</sub> because the periplasmic enzymes required for the breakdown of GlcNAc<sub>6</sub> were found to be downregulated at the mRNA level. We also showed that natural competence can be induced only by GlcNAc<sub>6</sub>, not GlcNAc<sub>2</sub>, because the expression of competence genes was significantly higher in the presence of GlcNAc<sub>6</sub> compared to GlcNAc<sub>2</sub>. Moreover, this might be an indication that GlcNAc<sub>2</sub> and GlcNAc<sub>6</sub> were detected by different receptors. Therefore, we speculate that GlcNAc<sub>2</sub>-dependent activation of ChiS and GlcNAc<sub>6</sub>-dependent activation of TfoS might be crucial for the induction of natural competence in <i>V. parahaemolyticus</i> through the upregulation of the master competence regulator TfoX.
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