Sequences and evolutionary analysis of mouse 5S rDNAs.

We selectively amplified the spacer regions of genes for mouse 5S ribosomal RNA (rRNA), which are tandemly repeated, by the PCR method, using primers specific to the two ends of the coding region for 5S rRNA. Fragments of approximately 1.6 kb were amplified from DNA from the BALB/cCrSlc mouse (Mus musculus domesticus), the SM/J mouse (M. m. domesticus), the MOA mouse (M. m. musculus) and the SEG mouse (M. spretus). These fragments were cloned into an appropriate plasmid vector, and two clones representative of each of the four strains were sequenced. The sequences were GC rich (> 60%) and contained a high proportion of very simple repetitive motifs, such as (TG)n and (ATCC)n, which accounted for the intra- and intergenomic length heterogeneity. Excluding such polymorphic regions and neglecting small insertions or deletions, we estimated the sequence divergence between clones. Sequence divergence within a genome averaged 0.26%, and the divergence between individuals of the same subspecies, between subspecies, and between species was 0.44%, 0.62%, and 1.73%, respectively. The results indicate that the spacer region evolved rapidly but with a reduction in heterogeneity within each genome, as a result of certain, as yet unidentified, homogenization mechanisms. The results further suggest that the spacer regions of genes for 5S rRNA may provide good indicators for phylogenetic analysis of closely related species.