MOLECULAR ANALYSIS OF PHYA IN WILD-TYPE AND PHYTOCHROME A-DEFICIENT MUTANTS OF TOMATO

Tomato (Lycopersicon esculentum recently redesignated Solanum lycopers icum L.), an agronomically important crop plant, has been adopted as a model species complementary to Arabidopsis in which to characterize t he phytochrome family. Here we describe the cloning and molecular char acterization of the gene encoding the apoprotein of phytochrome A in w ild-type tomato and in the far-red-light-insensitive (fri(1) and fri(2 )) tomato mutants. The physical organization of this gene is similar t o that of other angiosperm phytochromes with the four exons of the cod ing region interrupted by three introns. The pool of transcripts is he terogeneous due to multiple transcription start sites and to three mod es of alternative splicing of the 5' leader. The leader in each altern ative transcript carries multiple upstream open reading frames of cons iderable length. At the genomic level, both fri mutants share an ident ical base substitution which changes a consensus AG/ to TG/ at the 3' end of the intron between exons 1 and 2. This mutation leads to aberra nt processing of the resultant pre-mRNA. While most mature transcripts retain the mutated intron, both cryptic splicing and exon skipping we re also detected. Cryptic splicing occurred both upstream and downstre am from the wild-type splice site. These observations are consistent w ith the hypothesis that exon definition in splicing of plant pre-mRNAs plays a secondary role to that of intron definition. Analysis of the frequency with which potentially functional phytochrome A apoproteins might be produced indicates that both fri(1) and fri(2) have less than 1% of the wild-type phytochrome A level.