GENETIC-ANALYSIS OF THE PHI-X174 DNA-BINDING PROTEIN

The phi X174 J protein is 37 amino acids in length and contains 12 bas ic residues. There are no acidic amino acids in the protein. The basic residues are concentrated in two clusters in the N-terminus which are separated by a proline-rich region. To investigate the morphogenetic functions of the J protein and possible mechanisms by which it may bin d DNA, a genetic analysis was conducted. Lysine --> leucine and argini ne --> leucine substitutions were generated within the basic amino aci d clusters. At least three substitutions were required to eliminate vi ability in vivo. Lethal mutants with three or four substitutions exhib it dominant lethal phenotypes, indicating that the mutant proteins ret ain enough function to interfere with productive assembly. In cells in fected with a dominant lethal mutant, noninfectious packaged particles were produced. Infectivity can be restored by second-site suppressors in the viral coat protein which disrupt polar interactions atop the t hreefold axis of symmetry in the capsid. The viability of strains cont aining compensating frameshift mutations within the proline-rich regio n suggests that only the proline residues in this segment are critical for efficient function. (C) 1997 Academic Press