Novel processing in a mammalian nuclear 28S pre-rRNA: tissue-specific elimination of an 'intron' bearing a hidden break site.

G J Melen

doi:10.1093/emboj/18.11.3107

Abstract

Splitting and apparent splicing of ribosomal RNA, both previously unknown in vertebrates, were found in rodents of the genus Ctenomys. Instead of being formed by a single molecule of 4.4 kb, 28S rRNA is split in two molecules of 2.6 and 1.8 kb. A hidden break, mapping within a 106 bp 'intron' located in the D6 divergent region, is expressed in mature ribosomes of liver, lung, heart and spleen, as well as in primary fibroblast cultures. Testis-specific processing eliminates the intron and concomitantly the break site, producing non-split 28S rRNA molecules exclusively in this organ. The intron is flanked by two 9 bp direct repeats, revealing the acquisition by insertion of a novel rRNA processing strategy in the evolution of higher organisms.

Highlights

The information flow from eukaryotic genes to functional RNA molecules involves a panoply of diverse and sometimes sophisticated RNA processing strategies
Co-homogenization of equal amounts of Ctenomys and rat liver pieces and subsequent RNA extraction resulted in rRNAs with a combined pattern of migration showing three bands in denaturing agarose gels (Figure 1A, lane 2), which indicates that rat 28S rRNA was not degraded during the preparation procedure as a consequence of an unspecific RNase activity present in Ctenomys tissues
We report here the discovery of two unexpected processing events in the large ribosomal subunit (LSU) rRNA in South American rodents of the genus Ctenomys

Summary

Introduction

The information flow from eukaryotic genes to functional RNA molecules involves a panoply of diverse and sometimes sophisticated RNA processing strategies. Four major categories of splicing mechanisms are recognized: splicing of group I, group II, nuclear mRNA and tRNA introns. In the two first classes the folded intron participates alone or aided by facilitating proteins in the splicing reaction. The third class is catalyzed by spliceosomes, and the fourth involves nucleases and ligases of protein nature. Group I introns are found in mitochondrial pre-mRNAs from Neurospora crassa, yeast and plants, pre-tRNAs from bacteria and plant chloroplasts, and pre-rRNAs from Tetrahymena thermophila, Physarum polycephalum and Pneumocystis carinii nuclei, yeast mitochondria and plant mitochondria and chloroplasts (Cech, 1990; Lambowitz and Belfort, 1993). Group II introns are found in bacteria (Ferat and Michel, 1993), fungal and higher plant mitochondria and in chloroplasts.

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