Evidence for interaction of 5.8S rRNA with the 5'- and 3'- terminal segments of Saccharomyces cerevisiae 25S rRNA.

The possible sites of 5.8S:25S rRNA interaction in Saccharomyces cerevisiae are investigated by blot-hybridization of in vivo 32P-labelled 5.8S rRNA with restriction fragments from the 25S rRNA gene. Strong hybridization signals are obtained with fragments from the 5'-end (nucleotides 1 to 494) and the 3'-end (3066 to 3391) of the gene. The fragments from the remaining part of the gene are negative. A computer analysis of the known Saccharomyces cerevisiae 5.8S (Rubin 1973) and 25S (Georgiev et al. 1981) rRNA sequences show a markedly higher complementarity between 5.8S rRNA and the 5'- and 3'-terminal segments of 25S rRNA corresponding to the hybridization positive rDNA fragments. In accordance with the experimental and sequence analysis data, two alternative end-to-end base pairing models of possible 5.8S:25S rRNA binding are proposed. Both models imply that 5.8S rRNA connects the 5'- and 3'-terminal segments of 25S rRNA, but differ in the extent of preservation of the secondary structure typical of free 5.8S rRNA. It is suggested that the requirement for 5'- and 3'-end binding of 23S rRNA in Escherichia coli is conserved in evolution and that in eukaryotes 5.8S rRNA plays the role of joining together the two ends of L-rRNA molecules.