The cytidine repressor participates in the regulatory pathway of indole in Pantoea agglomerans

Indole, an important signal molecule in both intraspecies and interspecies, regulates a variety of bacterial behaviors, but its regulatory mechanism is still unknown. Pantoea agglomerans YS19, a preponderant endophytic bacterium isolated from rice, does not produce indole, yet it senses exogenous indole. In this study, a mutant of YS19-Rpr whose target gene expression was downregulated by indole was selected through mTn5 transposon mutagenesis. Using the TAIL-PCR technique, the mutation gene was identified as a cytR homologue, which encodes a cytidine repressor (CytR) protein, a bacterial transcription factor involved in a complex regulation scheme. The negative regulation of indole in cytR, which is equivalent to the mutation in cytR, promotes the expression of a downstream gene deoC, which encodes the key enzyme deoxyribose-phosphate aldolase in participating in pentose metabolism. We found that DeoC is one of the regulatory proteins of P. agglomerans that is involved in counteracting starvation. Furthermore, the expression of deoC was induced by starvation conditions, accompanied by a decrease in cytR expression. This finding suggests that the indole signal and the mutation of cytR relieve inhibition of CytR in the transcription of deoC, facilitating better adaptation of the bacterium to the adverse conditions of the environment.