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.