The Attack And Defence Mechanisms Of Pseudomonas Fluorescens W-6 And Its Siphoviridae Bacteriophage VW6S

The co-evolution of bacteria and bacteriophages is a long-term,dynamic and mutual adaptation process.In this process,bacteria and phages form a complex interaction called ‘the attack and defence mechanism'.It plays an important role in maintaining genetic diversity of the host and its bacteriophage,and is also one of the driving forces for generating new genes.In this study,the 21st-40 th co-evolution periods of Pseudomonas fluorescens W-6and its Siphoviridae bacteriophage VW6 S were used as materials to conduct a comprehensive genome analysis.In order to reveal the host-phage attack and defence mechanism,the resequencing data of W-6 and VW6 S in different periods analyzed,transcripts sequencing analysis of W-6 RNA during phage infection was carried out,and the changes in gene expression of host bacteria and the virus titer in different periods were compared.It was found that the host W-6 had 2 CRISPRs,and only one was located in the ORF encoding the nuclease(W-6?5259).The host contained two prophages.One was 41,297 bp showing 94% identity with phage VW6 S,and another with a length of 38,126 bp,had 87% similar with phage ?AH14a.The host contained970 Sam I methylation sites.Methylase digestion revealed that neither phage DNA nor host DNA had undergone methylation protection.The host bacteria had 226 SNPs in 25 genes during the co-culture process,involving 71 missense mutations base on the resequencing analysis.7 missense mutations can be inherited,and the most heritable mutations occurred in ORF W-6?1406,which encoding host specificity protein and showing 27 missense mutations.During the co-culture,there were 19 SNPs in 3 genes of VW6 S,all of which were nonsynonymous mutations.Among them,the VW6S?28(gp28)had 14 missense mutations,which encoding tail fiber domain protein.These gene mutations affected the adsorption capacity of the phage.It showed that the expression of some host's genes was affected based on the W-6 transcriptome sequencing analysis when the phage infected host.Compared with the differential expression of 8 genes and the number of bacteriophage virus titer in 11 different periods,the expression of W-6?1406,which encoding host specificity protein,was significantly down-regulated,effectively inhibiting the adsorption capacity of bacteriophage.However,there was no significant difference in the expression of CRISPR genes encoding nucleases.Within 20 periods of phage infecting the host,the host W-6 mutated its own host specificity protein to acquire immunity.The expression of host-specific genes was reduced to weaken the adsorption capacity of VW6 S during the adsorption stage.VW6 S adapted to the host-specific protein of W-6 by mutation of tail fiber domain protein.This study preliminarily revealed the attack and defence mechanism of W-6 and VW6 S,which is of great significance for further research on horizontal gene transfer and phage-host interaction mechanism.