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Genetic characterization of the hdrRM operon: a novel high-cell-density-responsive regulator in Streptococcus mutans

Justin Merritt^1,,

Lanyan Zheng^2,

Fengxia Qi^1,

¹ UCLA School of Dentistry, Department of Oral Biology, Los Angeles, CA 90025, USA
² China Medical University, Department of Microbiology and Parasitology, Shenyang, China
³ UCLA Molecular Biology Institute, Los Angeles, CA 90025, USA

Correspondence
Justin Merritt
justin-merritt{at}ouhsc.edu

Many species of bacteria can adhere to surfaces and grow as sessile communities. The continued accumulation of bacteria can eventually lead to the extremely high-cell-density environment characteristic of many biofilms or cell colonies. This is the normal habitat of the cariogenic species Streptococcus mutans, which normally resides in the high-cell-density, multispecies community commonly referred to as dental plaque. Previous work has demonstrated that the transcription of two separate bacteriocins can be activated by the high-cell-density conditions created through the centrifugation and incubation of cell pellets. In this study, we identified an uncharacterized two-gene operon that was induced >10-fold by conditions of high cell density. The genes of the operon encode a putative transcription regulator and a membrane protein, which were renamed as hdrR and hdrM, respectively. A transcription fusion to the hdrRM operon confirmed its induction by high cell density. Mutation of hdrM abolished bacteriocin production, greatly increased natural competence, reduced the growth rate, and severely affected biofilm formation. Interestingly, no obvious phenotypes were observed from a non-polar mutation of hdrR or mutations affecting the entire operon. These data suggest that the hdrRM operon may constitute a novel regulatory system responsible for mediating a cellular response to a high-cell-density environment.

These authors contributed equally to this work.

Present address: University of Oklahoma Health Sciences Center BRC364, 975 NE 10th St, Oklahoma City, OK 73104-5419, USA.

Present address: University of Oklahoma Health Sciences Center, College of Dentistry, Oklahoma City, OK 73104, USA.