Lambda phage cro repressor

cro repressor of coliphage lambda plays a vital role in the regulation of the phage life-cycle. It exerts control of phage functions at the transcriptional level by its ability to recognize and specifically bind the operators controlling the expression of the early genes of the phage. We have examined the interaction of cro with double and single-stranded DNA and RNA, and a synthetic lambda pseudo-operator, by monitoring the quenching of the tyrosyl fluorescence of cro repressor by added nucleic acid. cro repressor was found to bind single-stranded DNA with affinities similar to that seen for non-specific double-stranded DNA. It also binds synthetic RNA to a significant degree. The lambda pseudo-operator, which differs in three base-pairs out of 17 from OR3, the operator with the greatest affinity for cro, binds cro repressor more tightly than does non-specific DNA. The stability of the cro repressor-pseudo-operator complex as a function of salt concentration shows that the specific complex is stabilized by other factors in addition to the ionic interactions seen with cro repressor-non-specific DNA complexes.