We have shown that transcription of early genes in the gamma- related temperate coliphage HK022 is broadly similar to that of its relatives, but nevertheless differs in an interesting way. HK022 expresses the first gene of its pL operon, which encodes Nun, a highly specific transcription termination factor, in the presence of prophage repressor. It appears to do this by synthesizing a novel repressor-activated transcript that begins immediately downstream of the pL promoter. Since no other pL operon genes are expressed in the presence of repressor, transcripts must terminate after nun in the presence of repressor but proceed through terminators in its absence. The diffusible factors that we assume are involved in antitermination of transcription have not yet been identified. If any are encoded by HK022, their genes are not located in the usual place for antitermination genes in other lambdoid phages. HK022 and gamma both encode proteins that promote recombination between special DNA sequences called attachment sites. The mechanism of site-specific recombination in the two phages is very similar, but the sites and one of the proteins (the Int protein) are not interchangeable. In order to localize the determinants that distinguish these two recombination systems, we have determined the primary structure of the HK022 attachment sites and recombination proteins, and have compared them to the analogous gamma elements. We have also interchanged segments of the two phage attachment sites. This analysis shows that the critical determinants of the specificity difference are located in the central 50 bp (the core region) of the phage attachment site. Since Int protein binds to sequences within and outside of this segment, this finding suggests that the domains of Int that recognize the core region lie in the non-conserved regions of the two proteins, and that the domain(s) of Int that recognize the exterior regions lie in the conserved regions.
