Role of gene 8 product in morphogenesis of bacteriophage T3

The product of gene 8 (gp8) of T3 phage is one of the minor head proteins located at the phage head-tail junction. To determine the role of gp8, an amber (8-) and four temperature-sensitive mutants (ts8) were characterized by sedimentation analysis, polyacrylamide gel electrophoresis, and extract complementation. Neither DNA-containing particles nor empty particles were formed in cells infected with 8-. In addition, prohead assembly was greatly reduced. Prohead assembly was also blocked in cells infected with all ts8 mutants at 42 degrees and with some ts8 even at 37 degrees. Proheads containing gpts8 were converted to empty heads when cell lysates were treated with chloroform. The protein compositions of proheads showed that the minor head proteins, gp8, gp15, and gp16, were lost from proheads formed in cells infected with ts8, but these minor proteins were present in proheads formed in cells infected with double mutants of ts8 and 5- or 19-, which are defective in DNA synthesis or DNA maturation, respectively. In vitro complementation experiments suggested that a ts mutation in gene 8 affected not only DNA packaging but also subsequent assembly steps. From these results, it is concluded that gp8 plays multiple roles in T3 phage morphogenesis, including prohead assembly, prohead stabilization, DNA packaging, and subsequent events.