Characterization of the sat operon in Streptococcus mutans: Evidence for arole of Ffh in acid tolerance

An essential protein translocation pathway in Escherichia call and Bacillus subtilis involves the signal recognition particle (SRP), of which the 54-k Da homolog (Ffh) is an essential component. In a previous study, we found t hat a transposon insertion in the ylxM-ffh intergenic region of the designa ted secretion and acid tolerance (sat) operon of Streptococcus mutans resul ted in an acid-sensitive phenotype. In the present study, we further charac terized this genomic region in S. mutans after construction of bonafide sat operon mutants and confirmed the role of the SRP pathway in acid resistanc e. Northern blot and primer extension analyses identified an acid-inducible promoter upstream of ylxM that was responsible for upregulating the coordi nate expression of all five genes of the sat operon when cells were grown a t acid pH. Two constitutive promoters, one immediately upstream of satD and one just 3' to the acid-inducible promoter, were also identified. Except f or Ffh, the functions of the sat operon gene products are unknown. SatC, Sa tD, and SatE have no homology to proteins with known functions, although Yl xM may function as a transcriptional regulator linked to genes encoding SRP pathway proteins. Nonpolar mutations created in each of the five genes of the sat locus resulted in viable mutants. Most striking, however, was the f inding that a mutation in ffh did not result in loss of cell viability, as is the case in all other microbial species in which this pathway has been d escribed. This mutant also lacked immunologically detectable Ffh and was se verely affected in resistance to acid. Complementation of the mutation resu lted in restoration of acid tolerance and reappearance of cytoplasmic Ffh. These data provide evidence that the SRP pathway plays an important role in acid tolerance in S. mutans.