Temperate bacteriophages possess a molecular switch, which regulates the lytic
and lysogenic growth. The genomes of the temperate telomere phages N15, PY54
and ɸKO2 harbor a primary immunity region (immB) comprising genes for the
prophage repressor, the lytic repressor and a putative antiterminator. The
roles of these products are thought to be similar to those of the lambda
proteins CI, Cro and Q, respectively. Moreover, the gene order and the
location of several operator sites in the prototype telomere phage N15 and in
ɸKO2 are also reminiscent of lambda-like phages. By contrast, in silico
analyses revealed the presence of only one operator (OR3) in PY54. The
purified PY54 Cro protein was used for EMSA studies demonstrating that it
exclusively binds to a 16-bp palindromic site (OR3) upstream of the prophage
repressor gene. The OR3 operator sequences of PY54 and ɸKO2/N15 only differ by
their peripheral base pairs, which are responsible for Cro specificity. PY54
cI and cro transcription is regulated by highly active promoters initiating
the synthesis of a homogenious species of leaderless mRNA. The location of the
PY54 Cro binding site and of the identified promoters suggests that the lytic
repressor suppresses cI transcription but not its own synthesis. The results
indicate an unexpected diversity of the growth regulation mechanisms in
lambda-related phages