Isolation of an alfalfa histone H3 gene: structure and expression.

A histone H3 gene was isolated from a dicotyledonous plant, alfalfa (Medicago sativa). The sequence analysis of this gene revealed no obvious GC preference in its codon usage. Apart from containing most of the typical consensus sequences found in both animal and plant histone genes, the alfalfa H3 gene exhibits distinct structural features such as (1) the unusual location of two GATCC motifs in its 5' flanking sequence, (2) the existence of a CGCGGATC on the nonsense strand at position -232, (3) the existence of a long palindromic structure, and (4) several polyadenylation signal-like sequences in the 3' flanking region. There are about 160 copies of histone H3 gene in alfalfa tetraploid genome.Using the alfalfa H3 gene as a probe to study the pattern of histone H3 transcripts in the alfalfa, we found that the H3 RNAs are undetectable in leaves, more in stems than in roots, and highest in somatic embryos. Moreover, the RNA products of H3 genes in all alfalfa tissues tested show unusually long nontranslated region compared to those of animal histone genes. An additional high molecular weight species of H3 transcript was detected only in somatic embryos.