Translational control of discoidin lectin expression in drsA suppressor mutants of Dictyostelium discoideum.

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Abstract
Genetic analysis in Dictyostelium discoideum has identified regulatory genes which control the developmental expression of the discoidin lectin multigene family. Among these, the drsA mutation is a dominant second-site suppressor of another mutation, disB, which has the discoidinless phenotype. We now demonstrate a novel mechanism by which the drsA allele exerts its suppressive effect on the disB mutation. Interestingly, drsA does not merely bypass the disB mutation and restore the wild-type pattern of lectin expression. Rather, drsA mutant cells have high levels of discoidin lectin synthesis during growth but do not express lectins during aggregation. In contrast, wild-type cells only express lectin protein during the aggregation period of development. Phenocopies of the drsA mutation show a pattern of discoidin expression similar to that seen in the bona fide mutant. These data suggest that there may be a mechanism of negative feedback, resulting from the high levels of discoidin lectin made during growth, which inhibits further discoidin lectin expression during development. Northern (RNA) analysis of developing drsA mutant cells shows that these cells contain high levels of discoidin mRNA, although no discoidin lectin protein is being translated from these messages. Therefore, expression of the discoidin gene family can be controlled at the level of translation as well as transcription.