**A Ribosomal Orphon Sequence fromXenopus laevisFlanked by Novel Low Copy Number Repetitive Elements**

Guimond, Alain
Moss, Tom

Centre de Recherche en Cancérologie et Départment de Biologie médicale l'Université Laval, Hôtel-Dieu de Québec, 11 Côte du Palais, G1R 2J6 Québec, Canada

^* Corresponding author

Received October 27, 1998; accepted December 14, 1998

^a Present address: Institute de Recherche en Biotechnologie, CNRC, 6100 Royalmount, Montreal, H4P 2R2, Canada

Biological Chemistry 380(2):p 167-174, February 1999.

Abstract

We have used a differential cloning approach to isolate ribosomal/non-ribosomal frontier sequences fromXenopus laevis. A ribosomal intergenic spacer sequence (IGS) was cloned and shown not to be physically linked with the ribosomal locus. This ribosomal orphon contained the IGS sequences found immediately downstream of the 28S gene and included an array of enhancer repetitions and a non-functional spacer promoter. The orphon sequence was flanked by a member of the novel ‘Frt' low copy repetitive element family. Three individual Frt repeats were sequenced and all members of this family were shown to lie clustered at two chromosomal sites, one of which contained the ribosomal orphon. One of the Frt elements contained an insertion of 297 bp that showed extensive homology to sequences within at least three otherXenopusgenes. Each homology region was flanked by members of the T2 family of short interspersed repetitive elements, (SINEs), and by its target insertion sequence, suggesting multiple translocation events. The data are discussed in terms of the evolution of the ribosomal gene locus.