AN INTRAGENIC SUPPRESSOR OF THE ARABIDOPSIS FLORAL ORGAN IDENTITY MUTANT APETALA3-1 FUNCTIONS BY SUPPRESSING DEFECTS IN SPLICING

The Arabidopsis floral organ identity gene APETALA3 (AP3) specifies th e identity of petals and stamens in the flower. In flowers mutant for the temperature-sensitive ap3-1 allele, the petals and stamens are par tially converted to sepals and carpels, respectively, ap3-1 contains a single nucleotide change in the AP3 gene that alters both an amino ac id in the AP3 protein and the 5' splice consensus site for intron 5. S urprisingly, the Ap3-1 mutant phenotype is not due to the missense mut ation but instead is due to defects in splicing; specifically, exon 5 is frequently skipped by the splicing machinery at the restrictive tem perature. In a screen for suppressors of ap3-1, we isolated an intrage nic suppressor, ap3-11, that functions to suppress the splicing defect s of ap3-1. Using a reverse transcriptase-polymerase chain reaction as say, we demonstrate that the percentage of full-length exon 5-containi ng AP3 RNAs correlates with the phenotype of the flowers in both ap3-1 and ap3-11. Rather surprisingly, the ap3-11 suppressor mutation is lo cated in intron 4. One model explaining the function of ap3-11 is that the ap3-11 suppressor creates a novel branch point sequence that caus es exon 5 to be more frequently recognized by the splicing machinery. The identification of such a suppressor strongly suggests that exon-sc anning models of intron-exon recognition are operative in plants.