We investigated the molecular basis of a soybean Kunitz trypsin inhibitor (KTi) gene mutation that prevents the accumulation of Kunitz trypsin inhibitor protein during seed development. We found that mRNA encoding the major Kunitz trypsin inhibitor protein (KTi3 mRNA) is reduced at least 100-fold in null (KTi-) embryos but that KTi3 gene transcriptional activity is similar in Kunitz trypsin inhibitor producing embryos (KTi+) and in KTi-embryos. We sequenced the Kunitz trypsin inhibitor KTi3 gene from both KTi3' and KTi3-lines and found that these genes differ by only three nucleotides (+481, +486, and +487) within the KTi3 coding region. Alteration of these nucleotides results in a frameshift within the KTi3-gene that causes premature termination during translation. Our results suggest that the KTi3-frameshift mutation results in KTi3-mRNA destabilization and leads to a drastic reduction in KTi3 mRNA prevalence.

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