Characterization of the small subunit of the terminase enzyme of the Bacillus subtilis bacteriophage SPP1.

The small subunit of bacteriophages SPP1 and SF6 terminase, G1P, share 71% identity clustered in three conserved segments (I, II, and III). Within segment I the helix-turn-helix DNA-binding domain was mapped, whereas segment III was found to be nonessential. For terminase activity, chimeric G1Ps, obtained by domain swapping between gene 1 of SPP1 and the SF6 origin (Chi1 to Chi4), were purified. The chimeric proteins behave in all respects similarly to the G1P of SPP1 or SF6. The major determinant for G1P:G1P interactions was found to lie within segment II. We showed that a G1P derivative (G1P*) lacking the 62 N-terminal residues (segment I), and Chi1 lacking the 45 C-terminal residues (segment III) interact with G1P. The N-terminal domain of G1P is necessary for terminase subunit assembly, because the large subunit of the terminase (G2P) interacts only with G1P and Chi1, but fails to do so with G1P*. These results suggest that segment III and the extended C-terminal part of SPP1 G1P do not play a major role in DNA recognition and that G1P recognizes an extended nucleotide sequence and DNA structure.