Transcriptional and Posttranscriptional Control of Phaseolin and Phytohemagglutinin Gene Expression in Developing Cotyledons of Phaseolus vulgaris.

The expression of phaseolin and phytohemagglutinin (PHA) in the developing cotyledons of a normal (Greensleeves) and a PHA-deficient (Pinto 111) cultivar of Phaseolus vulgaris was investigated. Phaseolin mRNA translational activity and abundance were present at similar levels in both cultivars. In contrast, PHA mRNA translational activity and abundance in Pinto 111 were less than 1% of the levels measured in Greensleeves. Using nuclear runoff assays, the transcription rate of phaseolin gene sequences was similar in both cultivars. The transcription rate of PHA gene sequences in Pinto 111 was only 20% of that measured in Greensleeves. Comparison of the transcription rates with the relative mRNA amounts measured in RNA blot hybridizations indicated that the normally expressed storage protein gene mRNAs were very stable with half-lives greater than several days. Because a low level of PHA gene transcription in Pinto 111 was measurable but no PHA mRNA accumulated, these results suggest that the PHA deficiency in Pinto 111 is due to a reduced transcription rate and possibly an instability of the mRNA.