Arginine Deiminase Gene Regulation in Streptococcus gordonii

Objectives: The arginine deiminase system (ADS) is one of two major ammonia-generating pathways in the oral cavity that play important roles in oral biofilm pH homeostasis and ecology. ADS gene expression in Streptococcus gordonii is inducible by arginine and subject to carbon catabolite repression. The purpose of this study was to examine the impact of pH and growth phase on ADS gene expression in S. gordonii, and to explore the role of a tRNA modifying enzyme encoded by the genetically-linked queA gene in ADS regulation.

Methods: Primer extension was used to map the transcriptional initiation site of queA and RT-PCR was employed to examine whether queA and arcR, which encodes the transcriptional activator of the ADS, were co-transcribed. A parcA-cat fusion construct was established in the wild-type and QueA-deficient strains of S. gordonii, and the expression of parcA in each strain was examined in different growth phases in a batch culture system and at different pH values in a chemostat.

Results: The queA gene was located 15-bp 5' to, and co-transcribed with, arcR from a promoter 5' to queA. ADS gene expression in the wild-type and queA strains of S. gordonii was induced during stationary phase and was up-regulated by growth at low pH. Induction by low pH was also retained in a CcpA-deficient strain of S. gordonii. QueA - and CcpA-deficient strains showed an overall increase in ADS gene transcription compared to the wild-type strain, but a corresponding increase in AD activity was not observed.

Conclusions: In addition to being regulated by carbohydrate and arginine, ADS expression in S. gordonii is sensitive to pH and growth domain . Moreover, a role for tRNA modification in transcriptional control of the ADS and a major contribution of post-transcriptional control of AD enzyme levels have been revealed.