Differential Spo0A-mediated effects on transcription and replication of the related Bacillus subtilis phages Nf and phi29 explain their different behaviours in vivo.

Members of groups 1 (e.g. 29) and 2 (e.g. Nf) of the 29 family of phages infect the spore forming bacterium Bacillus subtilis. Although classified as lytic phages, the lytic cycle of 29 can be suppressed and its genome can become entrapped into the B. subtilis spore. This constitutes an alternative infection strategy that depends on the presence of binding sites for the host-encoded protein Spo0A in the 29 genome. Binding of Spo0A to these sites represses 29 transcription and prevents initiation of DNA replication. Although the Nf genome can also become trapped into B. subtilis spores, in vivo studies showed that its lytic cycle is less susceptible to spo0A-mediated suppression than that of 29. Here we have analysed the molecular mechanism underlying this difference showing that Spo0A differently affects transcription and replication initiation of the genomes of these phages. Thus, whereas Spo0A represses all three main early promoters of 29, it only represses one out of the three equivalent early promoters of Nf. In addition, contrary to 29, Spo0A does not prevent the in vitro initiation of Nf DNA replication. Altogether, the differences in Spo0A-mediated regulation of transcription and replication between 29 and Nf explain their different behaviours in vivo.