Selection of DNA binding sites by regulatory proteins: the LexA protein and the arginine repressor use different strategies for functional specificity.

The DNA sequences in the operator sites of the arginine regulon and of the SOS regulon have been subject to a statistical analysis. A quantitative correlation is found between the statistics of sequence choice and the activity at individual operator sites in both systems, as expected from theoretical considerations [Berg & von Hippel, J.Mol.Biol. (1987) 193, 723-750]. Based on these correlations it is possible to predict the effect of various sequence mutations. There is a significant difference in the slopes of the correlation lines between sequence and activity for the two systems. From this difference it can be expected that individual point mutations in the ARG boxes will have a much smaller effect on activity than similar changes in the SOS boxes. This difference may be related to a strong cooperative activity at tandem ARG boxes while the binding at SOS boxes appears to be mostly noncooperative.