Truncated phenylalanine ammonia-lyase expression in tomato (Lycopersicon esculentum)

Southern blot analyses of genomic DNA fragments suggest there are five different classes of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) genomic sequence in tomato (Lycopersicon esculentum). Isolation and subsequent sequence analysis of three examples from genomic libraries reveal highly homologous coding sequences but also a surprisingly high frequency of single point mutations which would truncate protein synthesis. The nucleotide sequence for one of the genes (PAL5) encodes a normal polypeptide of 721 amino acids, interrupted by a 710-base pair intron in the codon for amino acid 139. In contrast, premature stop codons, 363 triplets from the end in PAL1 and 304 triplets from the end in PAL3 would result in substantially (51-43%) shorter polypeptides that are consistent with the protein polymorphism, recently reported in alfalfa (Jorrin, J., and Dixon, R. A. (1990) Plant Physiol. 92, 447-445) but ascribed to protein degradation. S1 mapping of the mRNA termini and polymerase chain reaction analysis of cDNA transcripts indicate that at least one of these truncated coding sequences is expressed, strongly suggesting that at least some of the shorter polypeptides constitute original gene products with a potentially important function.