THE ORGANIZATION OF HISTONE GENES IN ARTEMIA (5S-RNA, MESSENGER-RNA)

Histones are a group of five small, basic proteins that serve to organize eukaryotic DNA into its fundamental unit, the nucleosome. Histone gene expression is coordinated with a synchronous wave of DNA replication in newly hatched larvae of the brine shrimp, Artemia. Four recombinant phage containing histone genes, designated (lamda)BSh1, 2, 3, and 4, were isolated from a library of Artemia genomic DNA fragments. The restriction maps of (lamda)BSh1, 3, and 4 are identical except for one extra Eco RI site in (lamda)BSh4. Each recombinant phage hybridizes with five histone mRNAs ranging in size from 800 to 450 bases and unexpectedly also with 5S RNA. To locate the histone and 5S-related sequences, restriction fragments from the recombinants were probed with cloned Drosophila histone and 5S genes. The 5S-related sequences are in the center of each recombinant, flanked by histone genes. The histone gene order in Artemia is H1-H2A-H4-H3-H2B with one 5S RNA gene located between genes for histones H2B and H1. Hybridization kinetics have shown the number of histone genes to be about 100 copies per haploid genome. An identical number of copies was determined for a 1000 bp Acc I fragment containing the 5S gene but no histone genes. Four Eco RI fragments from (lamda)BSh4 were sub-cloned and used to probe restriction fragments of genomic DNA. Nearly every fragment predicted by the restriction pattern of (lamda)BSh4 was found in genomic DNA in a pattern that indicated tandem duplication in the genome of the DNA sequences cloned in this recombinant. Hybridization of the Acc I fragment and sub-cloned histone genes to identical fragments of Artemia genomic DNA suggests that genes for histones and 5S RNA are organized in the same 8.5 kb repeating structure.