Abstract
On the basis of the host ranges of the interference-resistant A25 mutants h1 and h2, which recombine with high frequency, five group A streptococcal strains carrying prophages interfering with wild-type A25 can be subdivided into three groups, a classification which is consistent with that made previously on a different basis [Behnke, D., and Malke, H. (1978) Virology 85, 118–128]. Mixed infection of appropriate lysogens with the two A25 mutants does not result in either rescue of the inhibited phage or suppression of the resistant one, suggesting that A25 does not form diffusible products which are involved in the interference process. Mutation h2 causing resistance to interference mediated by prophages P6240, P13234mo, and R58/14820 increases the sensitivity of A25 to P5004 interference and suppresses h1, which when present alone renders A25 resistant to the latter mechanism. Since A25h2 is still inhibited on an interference-deficient P5004-lysogenic strain which supports growth of A25, it is possible that P5004 determines two distinct interference mechanisms, one acting on A25 and the other on A25h2. Prophage-mediated interference and classical phage restriction/ modification are found to act side by side in strain 5004, the natural host of P5004.
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