Molecular Characterization and Phylogeny of U2AF Homologs in Plants

U2AF (U2 small nuclear ribonucleoprotein auxiliary factor) is an essential splicing factor with critical roles in recognition of the 3#-splice site. In animals, the U2AF small subunit (U2AF) can bind to the 3#-AG intron border and promote U2 small nuclear RNP binding to the branch-point sequences of introns through interaction with the U2AF large subunit. Two copies of U2AFencoding genes were identified in Arabidopsis (Arabidopsis thaliana; atU2AFa and atU2AFb). Both are expressed in all tissues inspected, with atU2AFa expressed at a higher level than atU2AFb in most tissues. Differences in the expression patterns of atU2AFa and atU2AFb in roots were revealed by a promoter::b-glucuronidase assay, with atU2AFb expressed strongly in whole young roots and root tips and atU2AFa limited to root vascular regions. Altered expression levels of atU2AFa or atU2AFb cause pleiotropic phenotypes (including flowering time, leaf morphology, and flower and silique shape). Novel slicing isoforms were generated from FCA pre-mRNA by splicing of noncanonical introns in plants with altered expression levels of atU2AF. U2AF homologs were also identified from maize (Zea mays) and other plants with large-scale expressed sequence tag projects. A C-terminal motif (named SERE) is highly conserved in all seed plant protein homologs, suggesting it may have an important function specific to higher plants.