Genetic studies have revealed a network of three unlinked regulatory genes that control the developmental expression of the family of endogenous lectins in Dictyosteliurn discoideurn.Mutations in the disA and dis5 loci have a null phenotype and do not express the discoidin I or I I lectins. The third mutation, drsA, is a second-site suppressor of the dis5 mutation, which restores expression of all lectin species. Cells carrying this mutation express the discoidin lectins during growth, which is in contrast to wild-type cells in which lectin synthesis is developmentally regulated. In addition to this basic level of genetic control, the conditions of growth dramatically influence the patterns of discoidin expression. Growth-phase wild-type cells do not express lectin if the cells are grown on plates in association with bacteria. However, wild-type cells growing in bacterial suspensions express high levels of lectin during growth. Synthesis of the discoidin lectins in growing cells is sensitive to the levels of extracellular cyclic adenosine monophosphate (CAMP) and folic acid. These results suggest that the drsA mutation renders cells insensitive to cAMP and/or folate and thus allows expression of lectin during growth.
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.
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.
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