Molecular analysis of a phytohemagglutinin-defective cultivar of Phaseolus vulgaris L.

The seeds of Phaseolus vulgaris cv. Pinto III are known to lack detectable amounts of phytohemagglutinin (PHA) and to accumulate very reduced levels of PHA mRNA compared with normal cultivars. Using PHA complementary-DNA clones and monospecific antibodies we analyzed cv. Pinto III genomic DNA and cotyledonary proteins synthesized both in vitro and in vivo. We detected genomic DNA sequences that hybridize with complementary-DNA clones for the two different classes of PHA polypeptides (PHA-E and PHA-L), at levels comparable to a normal bean cultivar. This indicates that the cv. Pinto III phenotype is not the result of a large deletion of the PHA structural genes. Messenger RNA isolated from cv. Pinto III developing cotyledons synthesizes in vitro very small amounts of a protein which is recognized by antibodies specific for PHA, and gives, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a single band with molecular weight similar but not identical to that of PHA-L polypeptides. This protein is also synthesized in vivo at a very reduced level, less than 1% compared with PHA in normal cultivars, and has mitogenic activity comparable to that of the PHA-L subunit, while it shows very weak erythroagglutinating activity. The initial steps in the synthesis and processing of this protein are identical to those already identified for PHA polypeptides. The cv. Pinto III protein could be either a PHA-L polypeptide whose synthesis is not affected by the mutation or a PHA-like lectin present normally at low levels in P. vulgaris.