Histone genes of Volvox carteri: DNA sequence and organization of two H3-H4 gene loci.

Two Volvox genomic clones each containing a pair of histone H3-H4 genes were sequenced. In both loci the H3 and H4 genes show outwardly divergent polarity, their coding regions being separated by short intercistronic sequences containing TATA boxes and a conserved 14-bp element. The 3' untranslated regions contain a characteristic motif with hyphenated dyad symmetry otherwise only found associated with animal histone genes. Derived amino acid sequences of histones H3 and H4 are highly conserved and identical between the two sets. The Volvox H3 genes both contain one intron whose relative position is shifted by one basepair. Sequence comparisons led to a new interpretation of intron sliding. The Volvox H3 gene structure combines the exon-intron organization of fungal H3 and vertebrate H3.3 genes with a termination signal typical for animal H3.1 genes. These features are discussed in view of histone gene evolution.