Organization, nucleotide sequence, and chromosomal mapping of a tandemly repeated unit containing the four core histone genes and a 5S rRNA gene in an isopod crustacean species

A tandemly repeated unit of 6553 bp containing a copy of the four core hist one genes H2B, H2A, H3, and H4, and also a 5S rRNA gene, was amplified by P CR from genomic DNA of the isopod crustacean Asellus aquaticus. The linkage between 5S rRNA genes and histone genes has been so far observed in only o ne other organism, the anostrac crustacean Artemia salina. The gene cluster was cloned and sequenced. The histone genes, in their 3' flanking region, have the interesting feature of possessing two different mRNA termination s ignals, the stem-loop structure and the AATAAA sequence. A part of the PCR product was used as a probe in FISH experiments to locate the gene cluster on an inter-individually variable number of chromosomes from 6 to 12 per di ploid cell, always in a terminal position and never associated with the het erochromatic areas. Fluorescence in situ hybridization (FISH) was also perf ormed on preparations of released chromatin and the reiteration level of th e gene cluster was determined as approximately 200-300 copies per haploid g enome.