Restriction and modification of bacteriophage SP10 DNA by Bacillus subtilis Marburg 168: stabilization of SP10 DNA in restricting hosts preinfected with a heterologous phage, SP18.

SP10 phage cannot propagate in Bacillus subtilis Marburg 168 containing the wild-type allele of either gene nonA or gene nonB. The latter gene codes for the intrinsic cellular restriction activity. SP10 DNA was degraded in nonB+ derivatives of Marburg 168. The degree of degradation depended upon the previous host in which SP10 was propagated. In the case of SP10 grown in B. subtilis W23 (a nonrestricting, nonmodifying bacterium), 90% of the phage DNA was hydrolyzed to acid solubles, and the residual acid-precipitable material was recovered as 0.5- to 1-megadalton fragments. In contrast, if SP10 was propagated in B. subtilis PS9W7 (a nonA nonB derivative of Marburg 168 that retains modifying activity), 40 to 50% of the input DNA was degraded to acid solubles, and most of the remainder was recovered as 15- to 20-megadalton fragments. In nonA+ nonB cells, SP10 DNA was conserved as unit-length molecules (ca. 80 megadalton). Prior infection of nonB+ cells with SP18 protected superinfecting SP10 DNA, even when rifampin or chloramphenicol was added before the primary infection. The data are discussed in terms of the following conclusions. (i) The nonB gene product of B. subtilis Marburg 168 is required for restriction of SP10 DNA. (ii) Some sites on SP10 DNA are sensitive to both the restricting and modifying activities, whereas other sites are nonmodifiable even though they are sensitive to the restriction enzyme. (iii) In some manner, SP18 antagonizes the action of the nonB gene product.