Characterization of phi 6 mutants that are temperature sensitive in the morphogenetic protein P12.

P12 is a morphogenetic protein necessary for the envelopment of the bacteriophage phi 6 nucleocapsid with the viral membrane. Gene 12 is located along with three other genes on the smallest chromosome of the virion. ts mutants in P12 were obtained by first characterizing the isoelectric focusing behavior of phi 6 proteins and then screening ts mutants of phi 6 that had previously been assigned to chromosome C for changes in the behavior of P12. In this manner, three independently isolated mutants were identified and were found to have morphogenetic consequences at restrictive temperatures similar to gene 12 nonsense mutants in nonsuppressor cells in that only unenveloped nucleocapsids were formed. When infected cells were labeled at restrictive temperature, 27 degrees, and then shifted to 21 degrees, normal phage particles were formed; however, the hydrophobic membrane proteins in the particles were not labeled, indicating that functional P12 must be present at the time of synthesis of the membrane proteins for them to assemble into virions or that the defective P12 leads the membrane proteins into a nonfunctional pathway.