nsd, a Locus That Affects theMyxococcus xanthusCellular Response to Nutrient Concentration

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Abstract
Expression of the previously reported Tn5lacΩ4469 insertion inMyxococcus xanthuscells is regulated by the starvation response. Interested in learning more about the starvation response, we cloned and sequenced the region containing the insertion. Our analysis shows that the gene fusion is located in an open reading frame that we have designatednsd(nutrient sensing/utilizing defective) and that its expression is driven by a σ70-like promoter. Sequence analysis of thensdgene product provides no information on the potential structure or function of the encoded protein. In a further effort to learn about the role ofnsdin the starvation response, we closely examined the phenotype of cells carrying thensd::Tn5lacΩ4469 mutation. Our analysis showed that these cells initiate development on medium that contains nutrients sufficient to sustain vegetative growth of wild-type cells. Furthermore, in liquid media these same nutrient concentrations elicit a severe impairment of growth ofnsdcells. The data suggest that thensdcells launch a starvation response when there are enough nutrients to prevent one. In support of this hypothesis, we found that, when grown in these nutrient concentrations,nsdcells accumulate guanosine tetraphosphate, the cellular starvation signal. Therefore, we propose thatnsdis used by cells to respond to available nutrient levels.