Filamentous Phages Affect Virulence of the Phytopathogen Ralstonia solanacearum

ϕRSS-type filamentous phages are frequently found integrated in Ralstonia solanacearum genomes and affect host virulence after infection. ϕRSS1, a known virulence-enhancing phage, was found to be a truncated form of a larger phage (designated as ϕRSS0; 7288 nt) integrated in strain C319. A 626-nt ϕRSS0 sequence missing in ϕRSS1 DNA contains a nucleotide sequence element attP, corresponding to dif of R. solanacearum. Thus, ϕRSS0 was integrated at a dif site, similarly to CTXϕ of Vibrio cholerae, which uses the host XerC/D recombination system. ϕRSS0 could integrate into both the chromosome and megaplasmid of the host genome. The extra region of ϕRSS0 also contained an open reading frame (ORF13) of 156 amino acids with sequence similarity to DNA-binding phage regulators. The ϕRSS0-attP is located within the ORF13-coding region; therefore, integration results in a truncation of the C-terminus of ORF13. ORF13 may function as a phage repressor for immunity, because strain C319 (a ϕRSS0 lysogen) is resistant to second infection by ϕRSS0. C319 is susceptible to ϕRSS1, thus ϕRSS1 (without ORF13) seems to be an escaped superinfective phage derived from ϕRSS0. The diversity and dynamic rearrangements of ϕRSS-type phages/prophages in R. solanacearum and their effects on host virulence are discussed.