Toward assigning helical regions in alignments of ribosomal RNA and testing the appropriateness of evolutionary models.

We suggest a nucleotide substitution model that takes correlation between base-paired nucleotides into account. The model includes the estimation of the transition-transversion ratio and allows inference of the shape parameter of a discrete gamma distribution to include rate heterogeneity. A Cox-test statistic, applied to a diatom ribosomal RNA alignment, shows that the suggested correlation model explains evolution of the stem region better than usual independence models. Moreover, the Cox-test procedure is extended to shed some light upon the problem of assigning helical regions in a secondary structure based alignment. This approach provides an estimate of the percentage of stem positions that do not appear to be correlated.