A2 Cro, the Lysogenic Cycle Repressor, Specifically Binds to the Genetic Switch Region of Lactobacillus Casei Bacteriophage A2

Virology. 1999 Sep 15;262(1):220-9. doi: 10.1006/viro.1999.9903.


Lysogenic induction of temperate bacteriophage A2 of Lactobacillus casei is controlled by the action of its cI and cro products at the phage operator region. Three 20-bp inverted repeated DNA segments (subsites O1, O2, and O3) and the two divergent (PL and PR) promoters were mapped within the 153-bp operator region. The A2-encoded Cro product is shown to be the functional homolog of lambda Cro. The binding of Cro to the three operator subsites is noncooperative and yields two discrete protein-DNA complexes of retarded migration in mobility shift assays. The Kapp value for the Cro-PL-PR DNA complex was estimated to be 6 nM. Cro shows a slightly higher affinity for O3 than for O1 and O2 subsites. The O3 subsite overlaps the -35 hexamer of the PL promoter, which directs cI expression. A Cro mutant protein, devoid of the last 12 residues (Cro*), allowed the assignment of the DNA-binding domain to the NH2 end of Cro. The C end enhances its affinity for the DNA and probably stabilizes bending induced by Cro.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Sequence
  • Binding Sites / genetics
  • DNA Footprinting
  • DNA, Viral / metabolism
  • Genes, Switch / genetics
  • Lactobacillus casei / genetics
  • Lactobacillus casei / virology*
  • Lysogeny / genetics*
  • Molecular Sequence Data
  • Operator Regions, Genetic / genetics*
  • Repressor Proteins / genetics
  • Repressor Proteins / metabolism*
  • Viral Proteins / genetics
  • Viral Proteins / metabolism*
  • Viral Regulatory and Accessory Proteins


  • DNA, Viral
  • Repressor Proteins
  • Viral Proteins
  • Viral Regulatory and Accessory Proteins
  • phage repressor proteins