SECONDARY STRUCTURE CONSERVATION OF THE U 3 SMALL NUCLEOLAR RNA INTRONS IN SACCHAROMYCES

Using a strategy based on PCR amplification of DNA and sequence analys is, we showed that the presence of introns with the characteristic fea tures of introns spliced in a spliceosome, in the U3A and U3B snoRNA g enes that code for the U3 small nucleolar RNA, is not a property restr icted to Saccharomyces cerevisiae. It is probably an ancient property of yeasts from the genus Saccharomyces. We detected the U3A and U3B sn oRNA genes in Saccharomyces bayanus and in a lager brewing yeast strai n. The U3A and U3B intronic sequences are highly conserved. Two additi onal ''U3B-like'' snoRNA genes were detected in the lager brewing yeas t Their intronic sequences show several differences, when compared to the U3B intronic sequence. However, despite the numerous mutations the intron secondary structure is conserved, especially, the central stru cture. This strongly suggests an important role of this central stem/l oop structure for spliceosome assembly and efficient splicing.