Abstract
Hypothetical proteins without defined functions are largely distributed in all sequenced bacterial genomes. Understanding their potent functionalities is a basic demand for bacteriologists. Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight of rice, is one of the model systems for the study of molecular plant pathology. One-quarter of proteins in the genome of this bacterium are defined as hypothetical proteins, but their roles in Xoo pathogenicity are unknown. Here, we generated in-frame deletions for six hypothetical proteins selected from strain PXO99(A) and found that one of them (PXO_03177) is required for the full virulence of this strain. PXO_03177 is conserved in Xanthomonas, and is predicted to contain two domains relating to polysaccharide synthesis. However, we found that mutation of this gene did not affect the production or modification of extracellular polysaccharides (EPSs) and lipopolysaccharides (LPSs), two major polysaccharides produced by Xoo relating to its infection. Interestingly, we found that inactivation of PXO_03177 significantly impaired biofilm formation and tolerance to sodium dodecyl sulfate (SDS), both of which are considered to play key roles during Xoo infection in rice leaves. These findings thus enable us to define a function for PXO_03177 in the virulence of Xoo. Furthermore, we also found that the global regulator Clp controls the transcription of PXO_03177 by direct binding to its promoter region, presenting the first cellular regulatory pathway for the modulation of expression of this hypothetical protein gene. Our results provide reference information for PXO_03177 homologues in Xanthomonas.