P re-mRNA splicing is one of the fundamental processes in constitutive and regulated gene expression in eukaryotes. During splicing, introns present in primary gene transcripts are removed and exons are ligated to produce translationally competent mRNAs. The basic mechanism of intron excision is similar in all eukaryotes. The reaction is mediated by the spliceosome, a large ribonucleoprotein (RNP) complex, which is assembled anew at each intron from small nuclear RNP particles (U-snRNPs) and numerous protein factors. Spliceosome assembly is a highly ordered and dynamic reaction, involving hydrolysis of several ATP molecules and many structural rearrangements Properties of plant introns The intron and exon organization of higher plant genes is similar to that of vertebrates In spite of these similarities, the requirements for intron recognition in plants differ from those in other eukaryotes, and plant cells generally fail to splice heterologous pre-mRNAs. The most important difference is a strong compositional bias for UA-or U-rich sequences in plant introns compared with those from yeast and vertebrates U12-type introns A minor class of nuclear pre-mRNA introns, referred to as U12-type or AT-AC introns (because they frequently start with AT and terminate with AC) have recently been described 3,13 . These introns contain different splice site and branch point sequences, and are excised by an alternative U12-type spliceosom
