PHASEOLIN NUCLEOTIDE-SEQUENCE DIVERSITY IN PHASEOLUS .1. INTRASPECIFIC DIVERSITY IN PHASEOLUS-VULGARIS

Most information about the molecular biology of phaseolin, the major s eed storage protein in Phaseolus vulgaris, has been obtained from the T-type phaseolin, which is characteristic of the Andean gene pool of t he 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 pr esence of two subtypes of S phaseolin, alpha and beta, depending on th e presence or absence, respectively, of a 27-bp direct repeat. These a re similar to the alpha- and beta-phaseolin subtypes found in the Ande an, T phaseolin; however, the additional 15-bp direct repeat also foun d in the T alpha-phaseolin gene type was apparently absent from the S alpha-phaseolin genes. The overall sequence identity was greater betwe en 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 pool s of P. vulgaris. Analysis of the putative amino acid sequence reveale d that both the 'Sanilac' phaseolin subtypes contained an additional m ethionine, however, not at the same site. This opens the possibility o f increasing the nutritionally limiting methionine level in phaseolin either through protein engineering or by screening accessions for reco mbinant phaseolin sequences that combine both substitutions.