Cis-trans position effect in Salmonella phage P22

Two nonsense mutants of Salmonella form a burst size in mixed infection of non-permissive bacteria which lies above that for non-complementing pairs but below that expected for complementation. The two mutants map closely together. To determine whether the two mutants reside in different genes or in the same gene, single bursts following mixed infection of non-permissive bacteria were analyzed for the relative frequency of wild type versus mutant phage. The results indicate that the two mutants lie in two neighboring genes. The mutants exhibit a cis-trans position effect characteristic of structural genes in bacterial operons.