Mutational analysis of the Escherichia coli spoT gene identifies distinct but overlapping regions involved in ppGpp synthesis and degradation.

The spoT gene of Escherichia coli encodes a guanosine 3',5'-bis(diphosphate) 3'-pyrophosphohydrolase (ppGppase) as well as an apparent guanosine 3',5'-bis(diphosphate) synthetase (designated PSII). To determine the regions of the SpoT protein that are required for these two competing activities, we analysed plasmid-borne deletion mutations for their ability to complement chromosomal mutations defective in each activity. We found that a region containing the first 203 amino acids of the 702-amino-acid SpoT protein was sufficient for ppGppase activity while an overlapping region containing residues 67-374 was sufficient for PSII activity. These data indicate that the catalytic sites involved in the two activities are separate but closely linked in the primary sequence of the SpoT protein. A ppGppase-defective delta 1-58 deletion mutant strain failed to synthesize ppGpp in response to nutrient limitation, also supporting the notion that PSII activity from wild-type SpoT does not increase in response to nutrient limitation. Using a strain lacking PSII activity but retaining ppGppase activity, we determined the contribution of the RelA protein (ppGpp synthetase I, PSI) to ppGpp synthesis following glucose starvation. We found that the RelA protein activity accounts for the initial burst of ppGpp synthesis at the onset of glucose starvation but that this source of synthesis is absent when amino acids are present during glucose starvation.