Analysis of a regulator involved in the genetic switch between lysis and lysogeny of the temperate Lactococcus lactis phage φLC3

摘要

Sequencing of a 1.3-kb fragment of DNA from the temperate Lactococcus lactis subsp. cremoris phage φLC3 revealed a pair of two divergently oriented ORFs, orf63 and orf286. The deduced amino acid sequence of the product of orf286 showed extensive homology to those of repressors of the temperate lactococcal phages r1t, Tuc2009 and BK5-T. A mutant with an amber mutation in orf286 gave rise to a clear plaque phenotype, indicating that this gene is involved in the lytic and lysogenic development of φLC3. Gel mobility shift assays showed that the partially purified orf26 protein bound specifically to the 224-bp intergenic region located between orf286 and orf63, and further characterization by DNase I footprinting analysis revealed that Orf286 protects two distinct sites within this region. Sequence analysis of the intergenic region revealed two putative, divergently oriented promoters, P_1 and P_2; orf286 and orf63 are probably transcribed from P_1 and P_2, respectively. In vivo analyses of P_1 and P_2 using β-galactosidase as a reporter enzyme in L. lactis showed that transcription from P_1 was repressed while transcription from P_2 was stimulated in the presence of the Orf286 protein. These results suggest a complex role for the Orf286 protein in regulating the genetic switch between lytic and lysogenic growth of φLC3.