A HIGHLY CONSERVED DNA-REPLICATION MODULE FROM STREPTOCOCCUS-THERMOPHILUS PHAGES IS SIMILAR IN SEQUENCE AND TOPOLOGY TO A MODULE FROM LACTOCOCCUS-LACTIS PHAGES

A highly conserved DNA region extending over 5 kb was observed in Stre ptococcus thermophilus bacteriophages. Comparative sequencing of one t emperate and 26 virulent phages demonstrated in the most extreme case an 18% aa difference for a predicted protein, while the majority of th e phages showed fewer, if any aa changes. The relative degree of aa co nservation was not homogeneous over the DNA segment investigated. Sequ ence analysis of the conserved segment revealed genes possibly involve d in DNA transactions. Three predicted proteins (orf 233, 443, and 382 gene product (gp)) showed nucleoside triphosphate binding motifs. Orf 443 gp showed in addition a DEAH box motif, characteristically found in a subgroup of helicases. and a variant zinc finger motif known from a phage T7 helicase/primase. Tree analysis classified orf 443 gp as a distant member of the helicase superfamily. Orf 382 gp showed similar ity to putative plasmid DNA primases. Downstream of ori 382 a noncodin g repeat region was identified that showed similarity to a putative mi nus origin from a cryptic S. thermophilus plasmid. Four predicted prot eins showed not only high degrees of aa identity (34 to 63%) with prot eins from Lactococcus lactis phages, but their genes showed a similar topological organization. We interpret this as evidence for a horizont al gene transfer event between phages of the two bacterial genera in t he distant past. (C) 1997 Academic Press.