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 bo th of the two major classes of snoRNAs (box C/D and box H/ACA) and are tran scribed as a polycistronic pre-snoRNA transcript from an upstream promoter. In contrast to vertebrate and yeast intron-encoded snoRNAs, which are proc essed from debranched introns by exonuclease activity, the particular organ isation of plant snoRNA genes suggests a different mode of expression and p rocessing. Here we show that single and multiple plant snoRNAs can be proce ssed from both non-intronic and intronic transcripts such that processing i s splicing-independent and requires endonucleolytic activity. Processing of these different snoRNAs from the same polycistronic transcript suggests th at the processing machineries needed by each class are not spatially separa ted in the nucleolus/nucleus.