Estimates by computer simulation of genetic distances from comparisons of RNAse A mismatch cleavage patterns.
The ribonuclease (RNase) protection assay (RPA) was evaluated as a method to estimate genetic distances among sequence variants of RNA viruses. The patterns of fragments generated, under different RPA conditions, by three sets of RNA sequence variants of known nucleotide sequence, were analyzed. Both the effectiveness of cleavage (i.e. the probability of cleavage in a certain heteroduplex) and its degree (i.e. in all the molecules in the assay or in a part of them) varied largely according to the nature of the mismatch. Probability and degree of cleavage were also dependent on distant sequence context effects. No correlation could be established between context and cleavage, so that the pattern of fragments in RPA cannot be unequivocally predicted from sequence information. Accordingly, nucleotide sequence differences between two sequence variants cannot be directly derived from RPA data. For all three sequence sets linear relationships were found between the number of non-shared fragments in the RPAs of two variants and their nucleotide sequence differences. Nevertheless, both linearity and the linear regression parameters varied largely according to the sequence set and according to RPA conditions, in a non-predictable way. Thus, under experimental conditions, RPA may not be as appropriate a method to estimate genetic distances between RNA sequences as simulation under an ideal model suggested. Possible ways to diminish the gap between the ideal model and the experimental procedure are proposed.