Characterization of the signal transduction pathways and cis-acting DNA sequence responsible for the transcriptional induction during growth and development of the lysosomal alpha-mannosidase gene in Dictyostelium discoideum.
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The lysosomal alpha-mannosidase gene in Dictyostelium discoideum is representative of a small group of genes that are expressed under two different conditions: 1) immediately upon removal of the bacterial food source from exponentially growing cells at < 5 x 10(5) cells/ml (which also initiates the developmental cycle), and 2) when the concentration of a secreted glycoprotein termed the prestarvation response factor (PSF) reaches a critical threshold in cultures growing at densities > 5 x 10(5) cells/ml. In this report we show that transcription of the alpha-mannosidase gene induced by starvation did not require protein synthesis in axenic wild-type strains, whereas protein synthesis was required for the transcriptional induction observed in response to PSF. Northern blot analysis was also done using mRNA from G alpha 1 and G alpha 2 gene disruption mutants. These genes encode subunits of heterotrimeric G proteins found at the cell surface in growing cells and cells early in development. The pattern of alpha-mannosidase gene expression was normal in these mutants as well as in mutants unable to produce the secreted glycoprotein conditioned medium factor or the cAMP receptor-1 protein. These genes have been shown to regulate the expression of many genes during early development. Promoter analysis studies identified a 145-base pair sequence element containing a TTG box which was required for alpha-mannosidase transcriptional induction under both starvation conditions and in response to PSF. The TTG box identified is an important regulatory element in the promoter of another prestarvation response gene, the discoidin I gamma gene. A ts mutant was found to misregulate the expression of both discoidin I and alpha-mannosidase expression at restrictive temperatures. Taken together these results suggest that the prestarvation response genes may be coordinately regulated possibly through the TTG box.
