Structural equivalence in the transcribed spacers of pre-rRNA transcripts in Schizosaccharomyces pombe

The structure of the internal transcribed spacer 2 (ITS2) in Schizosaccharo myces pombe was re-evaluated with respect to phylogenetically conserved fea tures in yeasts, features in other transcribed spacer regions as well as th e binding of transacting factors which potentially play a role in ribosomal maturation, Computer analyses and probes for nuclease protection indicate a very simple core structure consisting of a single extended hairpin which includes the interacting termini of the mature 5.8S and 25S rRNAs, Comparis ons with ITS2 sequences in greatly diverging organisms indicate that the sa me feature also can be recognized, This is especially clear in organisms th at contain very short sequences in which the putative structures are much l ess ambiguous, Diversity between organisms is the result of changes in hair pin length as well as the addition of branched helices, Protein binding and gel retardation studies with the S. pombe ITS2 further indicate that, as o bserved in the 3' external transcribed spacer (ETS) and ITS1 regions, the e xtended hairpin is not only the site of intermediate RNA cleavage during rR NA processing but also a site for specific interactions with one or more so luble factors. Taken together with other analyses on transcribed spacer reg ions, the present data suggest that the spacer regions all may act in a sim ilar fashion, not only to organize the maturing terminal sequences, but als o serve to organize specific soluble factors possibly acting with snoRNAs o r in a manner which is analogous with that of the free snoRNPs.