Molecular characterization and evolution of the repeating units of histone genes in Drosophila americana: coexistence of quartet and quintet units in a genome.

Quintet and quartet repeating units of the histone genes in Drosophila americana were cloned and characterized. Nucleotide sequence analysis of the units showed that a 3175 bp unit contained the core histone genes but lacked the H1 gene ('quartet unit') while a 5025 bp unit contained all five histone genes ('quintet unit'). Comparative analysis suggested that these repeating units diverged before the separation of D. americana and D. virilis. Multiple forms of H1 genes, differing by 5.8% of amino acids, were found in D. americana. The genomic organization of the histone gene family in D. americana was found to be very similar to that of D. virilis.