Analysis of the morphogenetic cluster and genome of the temperate Lactobacillus casei bacteriophage A2

Summary.

 The genes that encode the morphogenetic proteins of bacteriophage A2 are clustered and expressed as a single operon which originates a late transcript of more than 20 kb. This DNA stretch is analyzed in the context of the whole phage genome, which presents the following peculiarities: a) the head presents two major proteins that share their NH₂ termini, i.e.: both are translated from a single gene (orf5), b) these two proteins suffer a proteolytic maturation process before being incorporated into the capsid, rendering a 123 NH₂ terminal putative polypeptide that is postulated to be the scaffolding protein of the phage, c) similar maturation processes occur at the portal and tail length determination proteins, having all in common a Pho-Pho-Arg↓ sequence (where Pho stands for any hydrophobic amino acid) at the processing point, d) the genes encoding the subunits of the terminase (orf61 and orf2) are separated by the cohesive ends, e) two genes that might mediate lysogenic conversion (orf19 and orf22) have been identified and f) the genome presents a dispensable region (which covers at least 10 orfs, as judged from analysis of deletion mutants) that might be involved in maintaining its size between the packaging limits of the capsid.