Bacillus subtilis bacteriophage SPP1 terminase has a dual activity: It is required for the packaging initiation and represses its own synthesis

The B. subtilis bacteriophage SPP1 terminase, encoded by genes 1 and 2, is required for the initiation of headful packaging. The DNA segment to which gene 1 product (GIP) binds includes the pact and pacR sites and the late PL1 and PL2 promoters from which genes 1 to 7 are transcribed. When SPP1wt or SPP1sus115 (gene 6(-)) phages were used to infect a B. subtilis sup(0) strain, the gene 1 to 7 mRNA synthesis was reduced at late times of infection. This was not observed, however, when either chloramphenicol was added 7 min after infection with SPP1wt or when SPP1sus114 (gene 1(-)) or SPP1sus19 (gene 2(-)) were used to infect B. subtilis sup(0) cells. These results suggest that the terminase enzyme functions as a repressor of its own transcription. G1P and B. subtilis RNA polymerase (RP) bind to the pact segment, which contains the PL1 and PL2 promoter region. The binding of G1P to the pact site does not seem to exclude RP from the promoters, despite of the overlapping of their binding sites. It is likely that the terminase protein does not repress transcription by a mere steric hindrance of RP binding.