Characterization of the two H1 degrees-encoding genes from Xenopus laevis

We have analyzed the promoter and the coding sequences of the two homologous histone H1 degrees-encoding genes from Xenopus laevis, here termed H1 degrees-1 and H1 degrees-2. Both genes encode proteins of 193 amino acids and differ at just 16 amino-acid residues. Putative regulatory sequences identified in the promoter region are the same and are highly conserved. However, significant differences exist in the 5' untranslated regions (UTR) of the transcribed sequences of these two genes, such as several deletions in the 5'-UTR of the H1 degrees-2 gene in comparison with the H1 degrees-1 gene 5'-UTR. The 3'-UTR is a short sequence of about 200 bp which is unexpected compared with the long 3'-UTR of mammalian H1 degrees mRNA, but it is in the same size range as in avian H5 mRNA. Thus, the main differences between these two genes are observed in sequences potentially involved in the regulation of the H1 degrees gene expression such as the 5'-UTR. The two genes are expressed during embryogenesis and in several adult tissues. We discuss these findings in terms of the evolution of histone H1 degrees genes in vertebrates and the appearance of histone H5 in avian species. (C) 1997 Elsevier Science B.V.