Temperate bacteriophages possess a genetic switch which regulates the lytic
and lysogenic cycle. The genomes of the temperate telomere phages N15, PY54,
and ϕKO2 harbor a primary immunity region (immB) comprising genes for the
prophage repressor (cI or cB), the lytic repressor (cro) and a putative
antiterminator (q). The roles of these products are thought to be similar to
those of the lambda proteins CI (CI prophage repressor), Cro (Cro repressor),
and Q (antiterminator Q), respectively. Moreover, the gene order and the
location of several operator sites in the prototype telomere phage N15 and in
ϕKO2 are reminiscent of lambda-like phages. We determined binding sites of the
ϕKO2 prophage repressor CB and lytic repressor Cro on the ϕKO2 genome in
detail by electrophoretic mobility shift assay (EMSA) studies. Unexpectedly,
ϕKO2 CB and Cro revealed different binding specificities. CB was bound to
three OR operators in the intergenic region between cB and cro, two OL
operators between cB and the replication gene repA and even to operators of
N15. Cro bound exclusively to the 16 bp operator site OR3 upstream of the ϕKO2
prophage repressor gene. The ϕKO2 genes cB and cro are regulated by several
strong promoters overlapping with the OR operators. The data suggest that Cro
represses cB transcription but not its own synthesis, as already reported for
PY54 Cro. Thus, not only PY54, but also phage ϕKO2 possesses a genetic switch
that diverges significantly from the switch of lambda-like phages