| Streptococcus mutans is a Gram-positive facultative anaerobic bacteria. It is the main etiological agent in the development of dental caries. S. mutans adheres to the surfaces of teeth, which results in the demineralization of tooth surfaces. S. mutans is able to adapt to acidic environments, using glucose for acid production and sucrose to synthesise extracellular polysaccharides for adhesion to the tooth surfaces. The regulatory mechanism of acid adaption and the virulence factors required are not clear, despite much research having been performed.Small non-coding RNA (sRNA) in bacteria is research focus in recent years. The systematic search of sRNAs in bacteria combined computational methods with experimental approaches. sRNAs have been found to be growth phase dependent and vary in response to growth medium. They have been implicated in the stress response, such as oxygen, temperature, osmotic pressure, pH value. They are also closely related to bacterial virulence, such as pathogenicity and drug resistance. There is the prospect of broad applications of sRNAs, such as developing a novel ultrasensitive diagnostic system and in the design of novel potential therapeutics based on sRNA-complementary peptide nucleic acids (PNAs).The first part of this study was bioinformatic identification of sRNAs in S. mutans UA159Genome. The genome was analyzed with sRNA Predict, sRNASVM, SIPHT, and the website'Oral pathogens non-coding small RNA prediction on the prediction of Streptococcus mutans'. From the four methods of prediction, the total number of sRNAs is334, of which40sRNAs predicted by at least2software. We detected all40sRNAs predicted by at least2software by qRT-PCR, of which5sRNAs detected. The transcript level of L10-Leader was relatively high compared to the other sRNAs and the change was more significant in mid-logarithmic growth phase and stationary growth phase. So L10-Leader was chosen for the further study.The second part of this study was as follows:1. Identification of L10-Leader in S. mutans UA159. We detected L10-Leader by Northern blotting. The Northern blot result was consistent with the qRT-PCR result. The transcript level of L10-Leader was lower in the stationary growth phase. The levels of L10-Leader were gradually downregulated from logarithmic growth phase to stationary growth phase.2. L10-Leader in different clinical strains of S. mutans. The levels of L10-Leader were higher in strain1than strain2, regardless of they were in logarithmic growth phase or stationary growth phase. Levels of L10-Leader were gradually downregulated from logarithmic growth phase to stationary growth phase.3. Expression of L10-Leader under different environmental stresses. For acid shock assays, the levels of L10-Leader were consistently higher in pH5.5than in pH7.5, from1.5h to4.5h. There were no significant changes in levels of L10-Leader when the S. mutans strain UA159was grown in0.1%glucose or1%glucose TYE medium and0.5%sucrose or1%sucrose TYE medium.4. Expression of predicted target mRNAs of L10-Leader target in S. mutans UA159. We selected five possible target mRNAs of L10-Leader according to the bioinformatic prediction for different growth phases and acid shock assays. The results of qRT-PCR showed that the transcript levels of mRNAs were lower in the stationary growth phase than in the logarithmic growth phase. These changes were similar to those of L10-Leader. For acid shock assays, the changes of mRNAs were smaller and slower than L10-Leader. The levels of five target mRNAs were consistently higher in pH5.5than in pH7.5at3.5h and4.5h, which were similar to L10-Leader.In this study, we identified sRNA L10-Leader in S. mutans combined computational methods with experimental approaches. And we got some variation in expression levels of L10-Leader. Such studies of sRNAs are very helpful to our understanding of the biological behavior of S. mutans, especially its growth and stress response.
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