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

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Abstract
Two Volvox genomic clones each containing a pair of histone H 3 -H 4 genes were sequenced. In both loci the H 3 and H 4 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 H 3 and H 4 are highly conserved and identical between the two sets. The Volvox H 3 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 H 3 gene structure combines the exon-intron organization of fungal H 3 and vertebrate H 3.3 genes with a termination signal typical for animal H 3 .1 genes. These features are discussed in view of histone gene evolution.