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, w hereas segment III was found to be nonessential. For terminase activit y, chimeric G1Ps, obtained by domain swapping between gene I of SPP1 a nd 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 r esidues (segment I), and Chi1 lacking the 45 C-terminal residues (segm ent III) interact with G1P. The N-terminal domain of G1P is necessary for terminase subunit assembly, because the large subunit of the termi nase (G2P) interacts only with G1P and Chi1, but fails to do so with G 1P. These results suggest that segment III and the extended C-termina l part of SPP1 G1P do not play a major role in DNA recognition and tha t G1P recognizes an extended nucleotide sequence and DNA structure. (C ) 1998 Academic Press.