An essential protein translocation pathway in Escherichia coli and Bacillus subtilis involves the signal recognition particle (SRP), of which the 54-kDa homolog (Ffh) is an essential component. In a previous study, we found that a transposon insertion in the ylxM-ffh intergenic region of the designated secretion and acid tolerance (sat) operon of Streptococcus mutans resulted in an acid-sensitive phenotype. In the present study, we further characterized this genomic region in S. mutans after construction of bonafide sat operon mutants and confirmed the role of the SRP pathway in acid resistance. Northern blot and primer extension analyses identified an acid-inducible promoter upstream of ylxM that was responsible for upregulating the coordinate expression of all five genes of the sat operon when cells were grown at acid pH. Two constitutive promoters, one immediately upstream of satD and one just 3 to the acid-inducible promoter, were also identified. Except for Ffh, the functions of the sat operon gene products are unknown. SatC, SatD, and SatE have no homology to proteins with known functions, although YlxM 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 finding 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 described. This mutant also lacked immunologically detectable Ffh and was severely affected in resistance to acid. Complementation of the mutation resulted 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.Acid tolerance is regarded as an important determinant for survival of cariogenic bacteria in the biofilm on teeth (7,37,49.) The microorganisms in dental plaque are exposed to continual cycles of acid shock resulting from the formation of acid end products during metabolism of dietary carbohydrate by the acidogenic microflora. In vivo pH measurements have revealed that the intake of carbohydrates can drop the plaque pH from 7 to 4 in less than 20 min (21, 24, 52). In addition, frequent carbohydrate intake results in a lower pH of "resting plaque," which has been associated with increased dental caries. Early studies (41) demonstrated that high caries activity is associated with rapid plaque acidification as well as tolerance of low pH by the plaque microorganisms.