Effect of deoxyribopolymers and ribopolymers on the sensitivity of the cyclic-AMP receptor protein of Escherichia coli to proteolytic attack.

The cAMP receptor protein (CRP) is an allosteric protein in which binding of cAMP effects a conformational change with a consequent increased affinity for DNA. Unliganded CRP is relatively resistant to attack by a variety of proteases (trypsin, subtilisin, Staphylococcus aureus V8 protease, clostripain, chymotrypsin) which cleave cAMP-CRP, producing N-terminal cores which have lost DNA binding activity. Binding of double-stranded deoxyribopolynucleotides and calf thymus DNA by cAMP-CRP confers protection against attack by trypsin, subtilisin, S. aureus V8 protease, and clostripain. Such cAMP-CRP-DNA complexes remain sensitive to attack by chymotrypsin. Of the single-stranded deoxy- and ribopolynucleotides tested, only r(I)n and r(A)n gave significant protection against attack by these proteases (with the exception of chymotrypsin). Since the cutting sites for trypsin (Lys 130) and subtilisin (Leu 116) are not part of the C-terminal DNA binding domain, it would appear that binding of DNA may confer conformational changes on other regions of cAMP-CRP.