U3 SNORNA GENES WITH AND WITHOUT INTRON IN THE KLUYVEROMYCES GENUS - YEASTS CAN ACCOMMODATE GREAT VARIATIONS OF THE U3 SNORNA 3'-TERMINAL DOMAIN

The U3 snoRNA coding sequences from the genomic DNAs of Kluyveromyces delphensis and four variants of the Kluyveromyces marxianus species we re cloned by PCR amplification. Nucleotide sequence analysis of the am plification products revealed a unique U3 snoRNA gene sequence in all the strains studied, except for K. marxianus var. fragilis. The K. mar xianus U3 genes were intronless, whereas an intron similar to those of the Saccharomyces cerevisiae U3 genes was found in K. delphensis. Hen ce, U3 genes with and without intron are found in yeasts of the Saccha romycetoideae subfamily. The secondary structure of the K. delphensis pre-U3 snoRNA and of the K. marxianus mature snoRNAs were studied expe rimentally. They revealed a strong conservation in yeasts of (1) the a rchitecture of U3 snoRNA introns, (2) the 5'-terminal domain of the ma ture snoRNA, and (3) the protein-anchoring regions of the U3 snoRNA 3' domain. In contrast, stem-loop structures 2, 3, and 4 of the 3' domai n showed great variations in size, sequence, and structure. Using a ge netic test, we show that, in spite of these variations, the Kluyveromy ces U3 snoRNAs are functional in S. cerevisiae. We also show that S. c erevisiae U3A snoRNAs lacking the stem-loop structure 2 or 4 are funct ional. Hence, U3 snoRNA function can accommodate great variations of t he RNA 3'-terminal domain.