In both the sea urchin and Drosophila, these genes are repetitive and clustered. In Strongylocentrotus purpuratus, the genes for H4, H2B, H3, H2A, and H1 lie in a linear sequence. They are transcribed from the same DNA strand to form a polycistronic message, and are separated by DNA spacers of similar lengths. In Drosophila there are about 110 copies of the histone genes, and these are localized in a four-band region in the left arm of chromosome 2. The gene sequence is H3, H4, H2A, H2B, H1. Two genes are transcribed from one DNA strand, three from the other. The gene order in Xenopus laevis is the same as in Drosophila. In humans, histone genes show less clustering, and they are located on a variety of chromosomes (1, 6, 12, and 22). Notophthalmus viridescens (q.v.) has 600 to 800 copies of the histone gene repeat. Transcribing histone genes have been detected in loops at specific loci on lampbrush chromosomes 2 and 6. Most histone genes lack introns (q.v.). See Chronology, 1972, Pardue et al.; 1981, Gall et al.; Strongylocentrotus purpuratus.
Subjects: Genetics and Genomics.