ABSTRACT
About 80% of plant nuclear genes contain one or more introns (Goodall et al., 1991; Reddy, 2001). The removal of introns is an important step in the expression of most eukaryotic genes. The introns in precursor messenger RNAs (pre-mRNAs) are removed in the nucleus to produce functional mRNAs. Excision of introns takes place in a large multicomponent complex called the spliceosome by a two-step reaction involving successive transesterification reactions (Burge et al., 1999). In the first step, the 5’ splice site is cleaved to generate a 5’ exon and a lariat intermediate. In the second transesterification reaction the 3’ splice site is cleaved to generate ligated exons and release the lariat intron. Pre-mRNAs with multiple introns are often spliced in alternate patterns and produce structurally and functionally different proteins from the same gene (Brett et al., 2000; Lorkovic et al., 2000; Reddy, 2001). The combinatorial joining of exons by alternative splicing is an elegant way that most eukaryotes use to generate several distinct proteins from a single transcript. Recent large-scale studies involving the comparison of ESTs with corresponding gene sequences indicate that alternative splicing of pre-mRNAs accounts for a large proportion of proteomic complexity in multicellular eukaryotes. It is estimated that pre-mRNAs from about 38% of all human genes undergo alternative splicing (Brett et al., 2000). Alignment of genes on human chromosome 22 with available ESTs and cDNAs indicate that about 59% of the genes are alternatively spliced to produce two or more transcripts (Consortium, 2001). In C. elegans about 22% of genes, for which ESTs were found, showed alternative splicing.
