Abstract: Eukaryotic cell contains diverse populations of metabolically stable small nucleolar RNAs (snoRNA). They are involved in many aspects of rRNA processing and maturation, snoRNAs have been identified in a wide range of eukaryotes and are presumed to be ubiquitous. Most of the snoRNAs characterized are from vertebrate and yeast, in which more than 130 species have been defined but only about 10 species of plant snoRNAs have been identified. The main reason may lie in the difficulties involved in different methodologies in isolating plant nuclei and nucleoli in appropriate amounts and purity for biochemical analysis. Although the study of plant snoRNA started later and the number of plant snoRNA is less, comparing with other eukaryotes, plant snoRNAs possess at least two distinct specific properties: (a) The way of snoRNA gene transcription. U3 is the first snoRNA discovered and identified in both vertebrate and plant. Plant U3 snoRNA gene promoters are clearly distinct from their yeast and mammamia counterparts, in beth the essential cis elements and the determinants of RNA polymerase specificity. The genes encoding the U3 snoRNA in plant are transcribed by RNA poi. Ⅲ, but all other organisms studied to date are by RNA Dol. 11. Plant U3 gene contains the conserved promoter elements USE and TATA, in a poi. Ⅲ-specific spacing. (b) The clusters of snoRNA genes. In animal and yeast, a large number of snoRNA are encoded within the introns of protein- ceding genes. The introns contain only single sonRNA gene and their processing involves exonucleolytic release of the snoRNA from debranched intron lariats. In contrast, some U14 genes and other snoRNAs, including box C/D and ACA families in plant, are found in clusters and are expressed polycistronically. And in the first intron of rice hsp70 gene, six snoRNAs, including box C/D and ACA families, closely linked together. How transcription of snoRNAs is coordinated with the expression of nueleolar or ribosomal proteins and the nature of the processing machinery remain to be elucidated. These differences from yeast and vertebrates, in snoRNA gene transcription and gene organization, imply the characteristic pathway of the evolution of the plant snoRNA genes.

Key words: Plant, Small nucleolar RNA(snoRNA), Transcription, Gene organization