Gene organization and transcription of a late-expressed region of a Lactococcus lactis phage.

The lactococcal phage bIL41 belongs to the small isometric-headed phages of the 936 quasi-species and is resistant to the abortive infection determined by abiB. A 10.2-kb segment from this phage, in which late transcription is initiated, has been sequenced. Thirteen open reading frames (ORFs) organized in one transcriptional unit have been identified. The location of two of them and the structural features of the proteins they code for are evocative of terminase subunits. Five other ORFs specify proteins which are highly homologous to structural proteins from the closely related phage F4-1. By comparing the phage bIL41 sequence with partial sequences available for four related phages, we were able to deduce a chimerical phage map covering the middle- and a large part of the late-expressed regions. Phages from this quasi-species differ by the insertion or deletion of either 1 to about 400 bp in noncoding regions or an entire ORF. Transcription was initiated 9 min after infection at a promoter with a -10 but no -35 consensus sequence. Synthesis of a phage activator protein was needed for initiation of transcription. A large 16-kb transcript covering all of the late-expressed region of the genome was synthesized. This transcript gave rise to smaller units. One of these units most probably resulted from a RNase E processing.