Characteristic Analysis Of DNA Methylation In Streptococcus Thermophilus And Its Effect On The Expression Of Acidogenic Genes

Streptococcus thermophilus(S.thermophilus)is one of the important lactic acid bacteria species in the production of fermented dairy products,which has excellent production characteristics and probiotic properties.DNA methylation not only plays an important role as a part of the restriction modification system,but also affects the phenotypic characteristics of bacteria as an important way to regulate gene expression.In this study,18 strains of S.thermophilus with different acid-producing abilities were used as the research object,and their DNA methylation characteristics at the population level were described.By analyzing the DNA methylation in the promoter region of key acid-producing genes,and using RT-q PCR technology was used to verify the effect of DNA methylation on the expression of acid-producing genes in S.thermophilus.The main research results are as follows:(1)The 18 S.thermophilus strains all had high methylation characteristics,and the motifs had high diversity.At the genome-wide level,DNA methylation hotspots were concentrated in genes encoding DNA replication,transcription repair and RNA binding proteins.It was speculated that DNA methylation affected the growth and reproduction of strains.In addition,the study found that the occurrence of DNA methylation was related to the genetic evolution of the strain and affected the acid production capacity of the strain.(2)The 18 strains of S.thermophilus have abundant and diverse restriction modification systems,and type I restriction modification systems widely exist in the strains.A variety of methyltransferase coding genes annotated in S.thermophilus may be one of the reasons for the large amount of DNA methylation in the strain.(3)By analyzing the carbohydrate metabolism pathway and amino acid metabolism pathway of S.thermophilus,it can be seen that the motifs AAAYNNNNNNNTGT,TGANNNNNNTCC,GGANNNNNNTCA and ACANNNNNNRTTT are only modified in strains with high acid production ability.It is speculated that the methylation of the above four motifs may be as one of the signs to distinguish different acid production rates of strains.(4)By analyzing the promoter regions of the key acid-producing functional genes pyk and pep O,it can be seen that the distribution of DNA methylation in the above-mentioned regions is different among strains with different acid-producing abilities.The results of RT-q PCR showed that the m4 C methylation site was negatively correlated with the expression of the above key functional genes,while the m6 A motif was positively correlated with the expression of the above key functional genes.However,the study found that 62.50% of the 18 strains of S.thermophilus with high acid production could utilize galactose,but the RT-q PCR results showed that DNA methylation was not one of the factors affecting the galactose metabolism ability of the strains.In summary,DNA methylation can affect the expression of acidogenic genes pyk and pep O in S.thermophilus,thereby affecting the acidogenic phenotype of the strain,And some methylated motif may be one of the signs to distinguish the acid production characteristics of the strains.The reason for the difference in acid production ability of strains is not only caused by DNA methylation,and the regulation mechanism of DNA methylation on the phenotype of strains still needs to be further studied.