Plant dicistronic tRNA-snoRNA genes: a new mode of expression of the small nucleolar RNAs processed by RNase Z.

Small nucleolar RNAs (snoRNAs) guiding modifications of ribosomal RNAs and other RNAs display diverse modes of gene organization and expression depending on the eukaryotic system: in animals most are intron encoded, in yeast many are monocistronic genes and in plants most are polycistronic (independent or intronic) genes. Here we report an unprecedented organization: plant dicistronic tRNA-snoRNA genes. In Arabidopsis thaliana we identified a gene family encoding 12 novel box C/D snoRNAs (snoR43) located just downstream from tRNA(Gly) genes. We confirmed that they are transcribed, probably from the tRNA gene promoter, producing dicistronic tRNA(Gly)-snoR43 precursors. Using transgenic lines expressing a tagged tRNA-snoR43.1 gene we show that the dicistronic precursor is accurately processed to both snoR43.1 and tRNA(Gly). In addition, we show that a recombinant RNase Z, the plant tRNA 3' processing enzyme, efficiently cleaves the dicistronic precursor in vitro releasing the snoR43.1 from the tRNA(Gly). Finally, we describe a similar case in rice implicating a tRNA(Met-e) expressed in fusion with a novel C/D snoRNA, showing that this mode of snoRNA expression is found in distant plant species.