The cis-specificity of the Q-gene product of bacteriophage lambda.

A trp/lacW205 substitution, fused to the late region of bacteriophage lambda, provided a convenient assay for phage late gene expression in the presence or absence of lambda pQ. Comparison of lacZ expression from Q+ and Q- phages showed that late gene expression was markedly Q-dependent (263-fold difference). A cis/trans comparison of lambda pQ action showed a 180-fold difference in lacZ expression. The results suggest that pQ in only significantly active when supplied in cis to its site of action.