Phaseolin nucleotide sequence diversity in Phaseolus. I. Intraspecific diversity in Phaseolus vulgaris.
Most information about the molecular biology of phaseolin, the major seed storage protein in Phaseolus vulgaris, has been obtained from the T-type phaseolin, which is characteristic of the Andean gene pool of the species. In the work reported here, two cDNA clones for the S-type phaseolin representing the other major, Middle American gene pool were isolated and sequenced. Analysis of the DNA sequences revealed the presence of two subtypes of S phaseolin, alpha and beta, depending on the presence or absence, respectively, of a 27-bp direct repeat. These are similar to the alpha- and beta-phaseolin subtypes found in the Andean, T phaseolin; however, the additional 15-bp direct repeat also found in the T alpha-phaseolin gene type was apparently absent from the S alpha-phaseolin genes. The overall sequence identity was greater between the alpha or beta subtypes of different gene pools than between the alpha or beta subtypes within gene pools. This implies that the gene subtypes were formed prior to the formation of the two major gene pools of P. vulgaris. Analysis of the putative amino acid sequence revealed that both the 'Sanilac' phaseolin subtypes contained an additional methionine, however, not at the same site. This opens the possibility of increasing the nutritionally limiting methionine level in phaseolin either through protein engineering or by screening accessions for recombinant phaseolin sequences that combine both substitutions.