Self-cleaving transcripts of satellite DNA from the newt

Abstract

Satellite 2 of the newt, Notophthalmus viridescens, is a 330 bp tandemly repeated sequence scattered throughout the genome. Cytoplasmic transcripts homologous to satellite 2 are found in a variety of tissues. Most of the transcripts correspond precisely in length to the DNA repeat unit or to whole multiples of that repeat. We show here that dimer-sized satellite 2 transcripts, synthesized with SP6 RNA polymerase from a plasmid clone, undergo site-specific, self-catalyzed cleavage in vitro. The reaction proceeds at neutral pH and requires Mg++ but no other cofactor or energy source. The cleavage products have 5'-hydroxyl and 3'-phosphate groups, at least some of which are in the form of 2',3'-cyclic phosphates. In this respect the reaction resembles the self-cleavage of certain small, infectious RNAs found in plants. Furthermore, the in vitro cleavage of satellite 2 transcripts occurs within a sequence that is homologous to the conserved cleavage site of the infectious RNAs. The existence of monomer and multimer transcripts in the cell suggests that the monomer may arise by site-specific cleavage of long primary transcripts. However, the 5' end of the cellular monomer is 46 or 47 bases upstream of the in vitro cleavage site, suggesting that factors in the cell may modify the cleavage reaction.