5S ribosomal and U1 small nuclear RNA genes: A new linkage type in the genome of a crustacean that has three different tandemly repeated units containing 5S ribosomal DNA sequences

We investigated the 5S ribosomal RNA (rRNA) genes of the isopod crustacean Asellus aquaticus. Using PCR amplification, three different tandemly repeat ed units containing 5S rDNA were identified. Two of the three sequences wer e cloned and sequenced. One of them was 1842 bp and presented a 5S rRNA gen e and a U1 small nuclear RNA (snRNA) gene. This type of linkage had never b een observed before. The other repeat consisted of 477 bp and contained onl y an incomplete 5S rRNA gene lacking the first eight nucleotides and a spac er sequence. The third sequence was 6553 bp long and contained a 5S rRNA ge ne and the four core histone genes. The PCR products were used as probes in fluorescent in situ hybridization (FISH) experiments to locate them on chr omosomes of A. aquaticus. The possible evolutionary origin of the three rep eated units is discussed.