Nucleotide sequences of H1 histone genes from Xenopus laevis. A recently diverged pair of H1 genes and an unusual H1 pseudogene.

Four clones containing H1 histone gene sequences were previously isolated from a Xenopus laevis genomic library (1) and we now present the complete nucleotide sequences of these H1 genes and their flanking regions. Two of these genes code for minor H1 proteins, probably H1C, when expressed in the oocyte transcription/translation system and are present on clones with almost identical overall organization. However, at the nucleotide level these genes differ in showing base insertions and deletions, as well as substitutions. A third gene sequence which is more related to the major X. laevis H1A, corresponds to the 3' two thirds of an H1 gene. This gene has in place of a 5' coding region at least 1800 bp of apparently noncoding sequence, some of which is A-T rich. The junction does not correspond to the consensus sequence of an intron/exon boundary and therefore this H1 sequence is more likely to represent a pseudogene. Comparisons of the coding and flanking regions of these X. laevis H1 genes indicate the kind of differences which can occur among H1 subtypes within a species. A region of homology noted in the 3' noncoding portion of vertebrate histone genes is discussed in relation to the mechanism of termination of transcription.