SEQUENCE SIMPLICITY AND EVOLUTION OF THE 3'-UNTRANSLATED REGION OF THE HISTONE H1-DEGREES GENE

The H1 degrees gene has a long 3' untranslated region (3'UTR) of 1,125 nucleotides in the rat and 1,310 in humans. Analysis of the sequences shows that they have features of simple DNA that suggest involvement of replication slippage in their evolution. These features include the length imbalance between the rat and human sequences; the abundance o f single-base repeats, two-base runs and other simple motifs clustered along the sequence; and the presence of single-base repeat length pol ymorphisms in the rat and mouse sequences, Pairwise comparisons show n umerous short insertions/deletions, often flanked by direct repeats. I n addition, a proportion of short insertions/deletions results from le ngth differences in conserved single-base repeats. Quantification of t he sequence simplicity shows that simple sequences have been more acti vely incorporated in the human lineage than in the rodent lineage. The combination of in sertions/deletions and nucleotide substitutions alo ng the sequence gives rise to three main regions of homology: a highly variable central region flanked by more conserved regions nearest the coding region and the polyA addition site.