| Fluoride is extensively used for caries prevention. Its effect oncariogenic bacteria is one of important mechanisms for the caries prevention byfluoride. But long-term topical applications of high-concentration fluoridemay lead to growth of F-resistant strains. F-resistant strains depress the effectof topical applications of fluoride and are more difficult to be restrained than theparent strains. The appearance of F-resistant strains bring the new issue forcaries prevention by fluoride. Studying the F-resistant strains can make us knowthe changes of cariogenicity of S. mutans after fluoride -resistant mutationbeforehand, afford experimental evidence for using fluoride rationally andfinding new medication. Streptococcus mutans is recognized as cariogenic bacterium whosecariogenicity is related with acidogenic ability and acid resistance. It is reportedthat the acidogenic ability and acid resistance of S. mutans increased after S.mutans fluoride -resistant mutated. Acid resistance is that bacteria can continueproducing acid and performing its function under low pH value,which includesconstitutive acid tolerance and acid tolerance response. Its acid resistance isdecided by many genetic factors. Some acid-resistant genes of S. mutans havebeen separated and cloned now. In this report we study the genes: dgk and dltC.Dgk gene encodes Diacylglycerol kinase(DGKase). Yamashita found that asingle insertion of transposon Tn916 into the S. mutans GS5 chromosomeresulted that GS5 is sensitive to acidic pH. Nucleotide sequence analysis of thisinsert fragment indicated the presence of two open reading frames,ORF1 andORF2. Transposon inserted near the 3' terminus of ORF1. The deduced aminoacid sequences of ORF1 showed significant homology with the diacylgycerolkinase from Escherichia coli. The resultant transformant showed wild-typelevels of aciduricity as well as resistance to elevated temperatures and highosmolarity. These results suggest that the S. mutans homolog of diacylglycerolkinase appears to play an important role in aciduricity as well as otherenvironmental stress contitions.The dlt operon of S. mutans comprises four genes: dltA,dltB,dltC anddltD. DltC encodes D-alanyl-carrier protein (Dcp). Boyd inactivated the dltCgene of the dlt operon in S. mutans LT11 to create the mutant BH97LC.BH97LC was shown to be acid-sensitive, displaying a defective ATR and anincreased permeability to protons compared to wild-type LT11. It was presumedthat D-alanyl-LTA whose information needs Dcp act as cell membrane's naturalbarrier to H+. When dltC is inactivated, H+ enters cell interior to make pH ofcell interior decreased, so the aciduricity depressed.It is repored that The genotypes of F-resitant strains of S. mutans havebeen changed and may comprise several gene mutations,but idiographic part ofgene mutation is not known. In this study, we designed the primers according todgk and dltC's sequences of S. mutans from GenBank database to do PCR. Sowe got the dgk and dltC's sequences of the F-resitant strains. We made theacquired sequences connected with PGEM-T vector and sequenced them. Wecompared the sequences with those from GenBank database by blastn and foundthat dgk's sequence has 8 base to be changed and dltC's has 1. The results weresubmitted to American GenBank, and the accession numbers are DQ272517,DQ272518. Dgk and dltC's mutation is point mutation and the substitution maycause little or no change in the amino acid sequence of the protein or in theability of the protein to function normally. It is silent mutation.DNA vaccine is the great progress in human preventive medicine. It's theoryis making exogenous gene imported into host cell, inducing host immune systemto carry out immune reaction. The study on acid-resistant genes dgk and dltCprefers new direction for DNA vaccine and has important significance.
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