Splicing-independent processing of plant box C/D and box H/ACA small nucleolar RNAs.

Small nucleolar RNAs (snoRNAs) are involved in various aspects of ribosome biogenesis and rRNA maturation. Plants have a unique organisation of snoRNA genes where multiple, different genes are tightly clustered at a number of different loci. The maize gene clusters studied here include genes from both of the two major classes of snoRNAs (box C/D and box H/ACA) and are transcribed as a polycistronic pre-snoRNA transcript from an upstream promoter. In contrast to vertebrate and yeast intron-encoded snoRNAs, which are processed from debranched introns by exonuclease activity, the particular organisation of plant snoRNA genes suggests a different mode of expression and processing. Here we show that single and multiple plant snoRNAs can be processed from both non-intronic and intronic transcripts such that processing is splicing-independent and requires endonucleolytic activity. Processing of these different snoRNAs from the same polycistronic transcript suggests that the processing machineries needed by each class are not spatially separated in the nucleolus/nucleus.