EXPRESSION OF THE BACTERIOPHAGE-T4 DNA TERMINASE GENE-16 AND GENE-17 YIELDS MULTIPLE PROTEINS

The products of the bacteriophage T4 terminase genes 16 and 17 are kno wn to mediate cutting and packaging of concatemeric vegetative DNA. We show here that the larger of these genes, 17, yields multiple protein species. The complex expression of the T4 terminase genes includes ov erlapping transcripts, probably initiated from multiple promoters, RNA processing at certain preferred sites and translation initiation from multiple ribosome binding sites (RES). Translation initiation from th ese RES may be modulated by inverted repeat (IR) sequences whose foldi ng can be predicted to differ in different RNA species. In T4 infected bacteria, genes 16 and 17 are probably co-transcribed from several ne ar-consensus late promoters upstream from gene 16, and processed at mu ltiple sites. Additional 5' ends of late transcripts are located downs tream from a near-consensus late promoter inside gene 17 and further d ownstream, unrelated to any known promoter consensus sequence. The gen e 17 transcripts that are initiated or cleaved internally contain RES for shorter open reading frames (ORFs) in the same frame as full-lengt h gene product (gp) 17 of 70 kDa. The truncated proteins, a 59-kDa gp1 7' and a 45-kDa gp17'', are synthesized from cloned gene 17 segments i n which the first gene 17 RES is deleted. Expression of gene 17 is dif ferent in BL21(DE3) or W3110[pACT7] host bacteria. The gp17' and gp17' ' proteins are predicted to contain one or more of the ATPase motifs t hat are common among large subunits of other phage terminases. They la ck a predicted single stranded (ss) DNA binding motif that is unique t he large terminase proteins in T4 gp17, and that has been implicated i n recognizing ssDNA regions in replicating and recombining T4 DNA dest ined to be packaged. We hypothesize that a truncated gene 17' is an ev olutionary precursor of the full-size T4 gene 17. Its function may hav e been maintained to allow processive packaging from double stranded ( ds) DNA ends.