Sequence analysis of the phage 21 genes for prohead assembly and head completion

Phage 21 is a temperate lambdoid coliphage, and its head-encoding genes, as well as those of phage lambda, are descended from a common ancestral phage. The head protein-encoding genes of phage 21 have been sequenced, confirming earlier genetic studies indicating that the head-encoding genes of 21 and lambda are analogous in location, size, and function. The phage 21 head-encoding genes identified (and their lambda analogues) include: 3(W), 4(B), 5(C), 6(Nu3), shp (D), 7(E), and 8(FII), respectively. An open reading frame, orf1, is analogous in position and shares some sequence identity with FI, a phage lambda gene involved in DNA packaging. The phage 21 major head protein, gp7, is predicted to have strong sequence identity (65%) with the lambda major capsid protein, gpE, including amino acids known to be important for capsid form determination. The nested genes 5/6 of phage 21 and C/Nu3 of lambda differ by several rearrangements including deletions and a triplication. The possibility that lambda genes C/Nu3 evolved from ancestal nested genes containing a triplication is discussed.