Effect of tra mutations on F factor-specified immunity to lethal zygosis

Hfr, F+, and F-prime cells are, unlike F- cells, insensitive to an excess of Hfr donor cells, indicating that there is an F factor mediated immunity to lethal zygosis (I1z). Results with Flac episomes carrying traJ, traS or various polar mutations in the tra region indicate that this immunity is independent of surface exclusion, of traJ control, and of all known genes within the tra operon. However, analysis of a series of strains with deletions in the F factor, extending from the right into the tra region, suggests that a gene for immunity to lethal zygosis is located within the tra region. We therefore conclude that I1z is genetically complex, and present a hypothesis to account for these results.