Molecular Basis of the Ribulose-l,5-Bisphosphate Carboxylase/Oxygenase Activase Mutation in Arabidopsis thaliana 1s a Cuanine-to-Adenine Transition at the 5'-Splice Junction of lntron 3

~ Analysis of the ribulose-1,5-bisphosphate carboxylase/oxygen- ase (Rubisco) activase gene and gene products from Arabidopsis thaliana wild-type plants and the Rubisco activase-deficient mutant strain showed that the rca mutation caused CT to be changed to AT at the 5'-splice junction of intron 3 in the six-intron pre-mRNA. Northern blot anaiysis, genomic and cDNA sequencing, and primer extension analysis indicated that the mutation causes inefficient and incomplete splicing of the pre-mRNA, resulting in the accu- mulation of three aberrant mRNAs. One mutant mRNA was iden- tical with wild-type mRNA except that it included intron 3, a second mRNA comprised intron 3 and exons 4 through 7, and the third mRNA contained exons 1 through 3. The C-to-A transition is consistent with the known mechanism of mutagenesis by ethyl methanesulfonate, the mutagen used to create the Rubisco acti- vase-deficient strain.