Streptococcus mutans ffh, a gene encoding a homologue of the 54 kDa subunit of the signal recognition particle, is involved in resistance to acid stress

The ability of Streptococcus mutans, a bacterial pathogen associated with d ental caries, to tolerate rapid drops in plaque ph (acidurance), is conside red an important virulence factor. To study this trait, Tn917 mutants of S, mutans Department of Oral strain JH1005 which display acid sensitivity hav e been isolated and partially Biology, University of characterized. In this paper, the characterization of one of these mutants, AS17, is reported. Pr eliminary sequence analysis revealed that the transposon insertion in AS17 occurred in the intergenic region of a two-gene locus which has been named sat for secretion and acid tolerance. This locus displays a high degree of homology to the ylxM-ffh operon of Bacillus subtilis. The sat(+) locus was cloned by complementation of a conditional Escherichia coli ffh mutant with an S. mutans genomic library. Sequencing of the complementing clone identi fied the intact ylxM and ffh genes as well as a partial ORF with homology t o the proU/opuAC gene of B. subtilis which encodes the binding protein of t he ProU/OpuA osmoregulated glycine betaine transport system. RNA dot blot e xperiments indicated steady-state levels of ffh mRNA in the mutant that wer e approximately eightfold lower compared to parental levels. This suggests a partial polar effect of the sat-1::Tn917 mutation on ffh expression. Upon acid shock (pH 5), wild-type ffh mRNA levels were found to increase approx imately four- to eightfold compared to unstressed (pH 7.5) levels. Mutant l evels remained unaltered under the same conditions. Experiments designed to investigate the origins of the acid-sensitivity of the mutant revealed a l ack of an acid-adaptive/tolerance response. Assays of proton-extruding ATPa se (H+/ATPase) specific activity measured with purified membranes derived f rom acid-shocked AS17 showed twofold lower levels compared to the parent st rain. Also, AS17 was found to be unable to ferment sorbitol although it was able to grow in glucose and a variety of other sugar substrates. These fin dings suggest that Ffh may be involved in the maintenance of a functional m embrane protein composition during adaptation of 5. mutans to changing envi ronmental conditions.