Lysogenisation of Spiroplasma citri by a type 3 spiroplasmavirus.

A short-tailed polyhedral spiroplasmavirus, ai, with a plaque morphology typical of temperate phages and a linear double-stranded DNA genome which can circularise due to the presence of cohesive ends, was able to lysogenise Spiroplasma citri. Lysogenic sporoplasmas spontaneously released ai virus at a low level, and were immune to superinfection by the released virus, but not to infection by two serologically related viruses. These properties were retained following repeated cultivation in virus antiserum. Viral sequences were detectable in host DNA by probing with recombinant DNA clones of ai fragments in coliphage M13 vectors. All strains of S. citri examined contained a deleted form of ai integrated as a cryptic prophage. We propose that ai circularises and integrates into the host genome adjacent to the cryptic prophage by a site-specific recombination event. Evidence of a correlation between lysogenisation and the attenuation of pathogenicity is presented.