Mutational analysis of the bacteriophage P4 capsid-size-determining gene.

Satellite phage P4 (11,624 bp) depends on the morphopoietic genes (capsid, tail) and lysis genes of its helper phage P2 (33.5 kb) for its lytic development. In the morphopoietic process, P4 redirects the assembly pathway of large, P2 size, capsids (diameter = 60 nm) to yield smaller, P4 size, capsids (diameter = 45 nm), 1/3 in volume of that of its helper. The P4-specified capsid size determination is dependent on the function of the 27-kDa gpSid. To study the capsid size-determining function, we carried out a mutational analysis of the P4 sid gene. Use of a P4-derived genome of 29.1 kb (P461), which can be packaged only into large, P2 size, capsids allowed us to select P4 Sid- mutants. By DNA sequencing we characterized 25 P4 Sid- mutants, of which 10 contain base pair substitutions and 15 contain deletions. Both types of mutations are clustered in separate locations within the sid gene. Our results suggest that the Sid polypeptide contains three distinct functional domains.