CHARACTERIZATION OF THE 2 H1-DEGREES-ENCODING GENES FROM XENOPUS-LAEVIS

We have analyzed the promoter and the coding sequences of the two homo logous histone H1 degrees-encoding genes from Xenopus laevis, here ter med H1 degrees-1 and H1 degrees-2. Both genes encode proteins of 193 a mino acids and differ at just 16 amino-acid residues. Putative regulat ory sequences identified in the promoter region are the same and are h ighly conserved. However, significant differences exist in the 5' untr anslated 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 se quence of about 200 bp which is unexpected compared with the long 3'-U TR of mammalian H1 degrees mRNA, but it is in the same size range as i n avian H5 mRNA. Thus, the main differences between these two genes ar e observed in sequences potentially involved in the regulation of the H1 degrees gene expression such as the 5'-UTR. The two genes are expre ssed during embryogenesis and in several adult tissues. We discuss the se findings in terms of the evolution of histone H1 degrees genes in v ertebrates and the appearance of histone H5 in avian species. (C) 1997 Elsevier Science B.V.