A novel gene organization: intronic snoRNA gene clusters from Oryza sativa

Based on the analysis of structural features and conserved elements, 27 novel snoRNA genes have been identified from rice. All of them belong to the C/D box-containing snoRNA family except for one that belongs to the H/ACA box type. The newly found genes fall into six clusters that comprise at least three snoRNA genes, and in one case as many as nine genes. Interestingly, four of the six clusters are located within the largest intron of a protein coding gene. The majority of intronic snoRNA gene clusters are simply formed by multiple copies of the same species of snoRNA gene that possess the identical functional elements. This implies a possible mechanism of duplication for the origin of repeating snoRNA coding regions in one intron. However, a few intronic snoRNA gene clusters consisting of different snoRNAs species were also observed. Polycistronic precursors from two independently transcribed clusters were demonstrated by RT-PCR and individual snoRNAs processed from the polycistronic precursors were positively determined by reverse transcription assay. Analyses of the intergenic spacers in the clusters showed that, in addition to a very high AT content, the processing signals in rice snoRNA polycistronic transcripts might be different from those of yeast. Our results demonstrate that, in both plants and mammals, numerous snoRNAs can be produced simultaneously from an mRNA precursor of a host gene despite the different arrangements. The intronic snoRNA gene cluster is a novel gene organization, which is so far unique to plants. The conservation of intronic snoRNA gene clusters in plants was further demonstrated by the study of a similar snoRNA gene organization in the first intron of a Hsp70 gene from wild rice and Zizania caduciflora.