The phi X174 J protein is 37 amino acids in length and contains 12 basic 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 bind DNA, a genetic analysis was conducted. Lysine --> leucine and arginine --> leucine substitutions were generated within the basic amino acid clusters. At least three substitutions were required to eliminate viability in vivo. Lethal mutants with three or four substitutions exhibit dominant lethal phenotypes, indicating that the mutant proteins retain enough function to interfere with productive assembly. In cells infected 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 threefold axis of symmetry in the capsid. The viability of strains containing compensating frameshift mutations within the proline-rich region suggests that only the proline residues in this segment are critical for efficient function.