In this article we will discuss about the processing of pre-snoRNA into mature RNA.
Small nucleolar RNAs are families of small RNAs involved in RNA metabolite and gene expression in eukaryotes. Their major function is in processing and modification of ribosomal RNA (rRNA). In plants, the 5.8S 18S and 28S rRNA of the cytoplasm ribosomes are produced by processing a precursor rRNA in the nucleolus.
Generally, Sno RNAs fall into mainly two classes: Box C/D Sno RNAs facilitate ribose methylation and box H/ACA SnoRNAs, which directs psuedouridylation. In nature, all SnoRNAs are closely associate with known type of nuclear proteins to organize as functional small nucleolar ribonucleoprotein particle (Fig. 3.1). Arabidopsis genome sequencing has revealed that there are 99 different box C/D SnoRNAs genes.
At least two box HACA SnoRNA genes (Sno R2 and Sno R5) have been found to linkage to box C/D Sno RNA genes. In the organization of Sno RNA genes, most plant Sno RNA genes are found in polycistronic clusters. Genomic analysis of rice revealed that most known rice SnoRNA genes are polycistronic and more than half of the rice clusters are intronic SnoRNA gene (Fig. 3.1).
The processing of polystronic and intronic pre-SnoRNA is accomplished by splicing or non-splicing process. Polycistrons are cleaved by endonuclease followed by exonucleolytic trimming. Binding of SnoRNA by core proteins leads to mature SnoRNA.
Extensive rRNA modification in plant (eukaryotic in general) compared with few modified nucleotides in prokarytes shows that ribosomal activity is drastically reduced when ribosylmethyl groups or pseudouredine depleted in yeast. Therefore modification of rRNA is indispensable for stability of functional rRNA structure.
Modification could influence interaction between ribosomal subunits and the binding of tRNA and mRNA. Thus, acts as fine tuner in the translational activity of the ribosome. In addition, it also acts as chaperon function in the folding of nascent rRNA or might stabilize rRNA tertiary structure.