Alternative splicing at the o2Italian locus in maize: one mutation, two proteins

Opaque2 is a basic leucine-zipper transcription factor that regulates the expression of several genes involved in the development of maize endosperm. Among the different mutations so far characterized, the o2Italian (o2It) allele has a genomic deletion of 10 bp that interrupts the open reading frame by a premature stop codon. Curiously, the o2It allele still encodes a long o2 isoform (o2It-L) that is recognized by an O2 carboxy-terminal antibody, evidencing that this protein is similar to the wild type isoform. To clarify this phenomenon, we characterized the transcripts generated from the o2It allele and found two transcripts that are produced in a 1:1 ratio. The first transcript carried the 10 bp deletion and matched the genomic sequence of the o2It allele, whereas the second one carried a 15 bp deletion as a result of a 3' alternative splicing site. The lack of five additional nucleotides restored the correct reading frame and explained the nature of the o2It-L polypeptide. The production of two distinct transcripts from the o2It allele was probably due to the recruitment of different splicing factors as suggested by in silico analysis of the o2It-mutated region. This finding evidences how the recruitment of splicing factors is tightly linked to nucleotide motives that should be present in correct neighboring contexts.