Mutational analysis of DNase I-DNA interactions: design, expression and characterization of a DNase I loop insertion mutant with altered sequence selectivity.

A mutant of bovine pancreatic DNase I containing two additional residues in a loop next to C173 has been expressed in Escherichia coli, purified and characterized biochemically. Modelling studies suggest that the inserted arginine and glutamate side chains of the modified loop sequence C173-R-E-G-T-V176 could contact the bases 3' to the cleaved bond in the major groove of a bound DNA, and that up to 10 bp could interact with the enzyme and potentially influence its cutting rate. The loop insertion mutant has an 800-fold lower specific activity than wild-type and shows overall cleavage characteristics similar to bovine pancreatic DNase I. Compared with the wild-type enzyme, the mutant shows a strongly enhanced preference for cutting the inverted repeat: (formula: see text) or close variants thereof. Unexpectedly for a minor groove binding protein, the preferred cutting sites in opposite strands are staggered by 1 bp in the 5' direction, causing the cleavage of a TA and a TT step, respectively. This finding demonstrates that the sequence context is relatively more important for the cutting frequency than the nature of the dinucleotide step of the cleaved bond, and clearly shows that base recognition is involved in determining the sequence selectivity of the mutant. The importance of the sequence 5' to the cleaved bond for the cutting rate suggests that the additional major groove contacts may require a distortion of the DNA associated with a higher energy barrier, resulting in an increased selectivity for flexible DNA sequences and a lower overall activity of the mutant enzyme.