Transcriptional Regulation of the Cellobiose Operon of Streptococcus mutans

Abstract

ABSTRACT The ability of Streptococcus mutans to catabolize cellobiose, a β-linked glucoside generated during the hydrolysis of cellulose, is shown to be regulated by a transcriptional regulator, CelR, which is encoded by an operon with a phospho-β-glucosidase (CelA) and a cellobiose-specific sugar p hospho t ransferase s ystem (PTS) permease (EII Cel ). The roles of CelR, EII Cel components, and certain fructose/mannose-PTS permeases in the transcriptional regulation of the cel locus were analyzed. The results revealed that (i) the celA and celB (EIIB Cel ) gene promoters require CelR for transcriptional activation in response to cellobiose, but read-through from the celA promoter contributes to expression of the EII Cel genes; (ii) the EII Cel subunits were required for growth on cellobiose and for transcriptional activation of the cel genes; (iii) CcpA plays little direct role in catabolite repression of the cel regulon, but loss of specific PTS permeases alleviated repression of cel genes in the presence of preferred carbohydrates; and (iv) glucose could induce transcription of the cel regulon when transported by EII Cel . CelR derivatives containing amino acid substitutions for five conserved histidine residues in two PTS regulatory domains and an EIIA-like domain also provided important insights regarding the function of this regulator. Based on these data, a model for the involvement of PTS permeases and the general PTS proteins enzyme I and HPr was developed that reveals a critical role for the PTS in CcpA-independent catabolite repression and induction of cel gene expression in S. mutans .