Comparative nucleotide sequence analysis of two types of larval beta-globin mRNAs of Xenopus laevis

The complete nucleotide sequence of the cDNA insert of the clone pXGL25 derived from the larval beta II-globin mRNA of Xenopus laevis has been determined. The sequence of 593 nucleotides represents part of the 5'nontranslated region, the coding region for 146 amino acids and the entire 3'nontranslated region. It diverges from the related larval beta I-sequence by 24.9% in the coding region. Alignment of the 5' and 3'nontranslated regions of the two related larval beta-sequences to maximum matching resulted in 31.2% and 46.7% divergence, respectively. Divergence between the corresponding adult and larval sequences considerably exceeds that of related larval sequences, suggesting that larval genes may have arisen by gene duplication prior to genome duplication. In contrast to mammalian beta-globin mRNAs, replacement and silent base substitutions are equally abundant, thus indicating less functional constraint on the larval Xenopus laevis beta-globin chains. The larval beta I- and beta II-globins diverge by 30.8% and show most variation in the alpha 1/beta 2-chain interaction sites.