The mannitol-specific enzyme II (mtlA) gene and the mtlR gene of the PTS of Streptococcus mutans

The phosphoenolpyruvate-dependent phosphotransferase system (PTS) is widely found among Cram-positive bacteria. It is the major source of carbohydrate transport in the dental pathogen Streptococcus mutans. The transported car bohydrates are fermented to produce large amounts of lactic acid which init iates dental caries. The authors have isolated the S. mutans gene for the m annitol-specific Enzyme II (Ell) component of the PTS, mtlA, and the adjace nt mtlR gene, which is located in the same operon. The mtlR gene is located between mtlA and the genes mtlF and mtlD. The nucleotide sequence of the m tlA and mtlR loci has been determined. The deduced mtlA gene product of S. mutans consists of 589 amino acids with a molecular mass of 62.0 kDa. It ex hibits similarity with the mtlA gene products from other organisms. However , the similarity between these proteins is generally restricted to the 470 amino-terminal residues of the S. mutans protein. This region would corresp ond to the EIICB domains of the PTS. The authors have previously shown that the S. mutans mtlF gene product exhibits 76.6% similarity to the carboxyl- terminal 143 amino acids of the Escherichia coil mtlA product and that the mflF gene encodes the EIIA domain of the PTS, Thus, the genes that encode t he EIICB and the EIIA domains are separated by approximately 2250 bp. In ma ny organisms, all of the Ell domains may be fused together to form one mole cule. The fact that these domains are separated by this distance in S. muta ns supports the hypothesis that various functional domains of the PTS have been rearranged during evolution. The sequence of the 119 carkoxyl-terminal amino acids of the S. mutans mtlA gene product also displays homology to t he carboxyl-terminal end of the EllB domain of various mannitol PTSs. Thus, this domain may have been duplicated in S, mutans during evolution of the operon, The mtlR gene is located in the same operon structure as mtlA but t hese loci are separated by an intragenic space. The precise 5' end of the m tlR locus cannot be determined either by in Vitro transcription-translation assays or based upon nucleotide sequence analysis because of the apparent lack of a ribosome-binding site preceding the gene. The deduced mtlR gene p roduct, which consists of approximately 650 amino acids with a molecular ma ss of 75.3 kDa, exhibits limited similarity to several potential transcript ional regulators. However, the exact function of this locus is currently un known.