Conservation of genome form but not sequence in the transcription antitermination determinants of bacteriophages lambda, phi 21 and P22.

Comparisons are made among DNA sequences upstream from terminators in both leftwards and rightwards early operons of related coliphages lambda, phi 21 and P22. These sequences include both left and right determinants of response to phage-coded antitermination proteins, "N", as well as the N structural genes themselves. Despite almost total disparity of DNA sequence, the three genomes can be discerned to include the same elements in the same order and spacing: downstream from the early left promoter are sequentially a site of recognition for host nusA protein, a dyad symmetry "nut" essential for N function in lambda, overlapping sites for processing of the transcript by RNAase III and then the N structural genes; downstream from the cro gene on the right are sites of nusA recognition and nut dyad symmetries homologous to those on the left. Because the N proteins of lambda, phi 21 and P22 do not for the most part complement each other, a specific site of N recognition has been postulated for each N-responding operon. The nut dyad symmetry qualifies as such a site, since the loop of the left dyad in lambda is marked by mutations that block N function leftwards, and since DNA sequences here show close homology between the loops of left and right dyads for each phage, but less if not little homology for different phages.