Developmental regulation of the alpha-mannosidase gene in Dictyostelium discoideum: control is at the level of transcription and is affected by cell density.

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Abstract
In Dictyostelium discoideum, there is a group of genes that are expressed following starvation and when exponentially growing cells reach high densities. We have examined the expression of one of these genes, alpha-mannosidase. Using an alpha-mannosidase cDNA probe in Northern (RNA) blot analysis, we have shown that the previously observed increase in alpha-mannosidase enzyme-specific activity during development is due to an increase in the levels of alpha-mannosidase mRNA. mRNA levels reach a maximum by 8 h of development and then begin to decline by 14 to 22 h. Using nuclear run-on analysis, we have found that this gene is regulated at the level of transcription. We also examined the effects of cell-cell contacts, cyclic AMP levels, and protein synthesis on expression of this gene and found that they were not critical in regulating its expression. However, cell density did play a major role in the expression of alpha-mannosidase. High cell density or the presence of buffer conditioned by high-density cells was sufficient to induce expression of alpha-mannosidase, indicating that this is one of the prestarvation response genes. Finally, the alpha-mannosidase gene was not expressed in aggregation-negative mutant strain HMW 404.

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