Variations in ppGpp levels in Rhodopseudomonas spheroides during adaptation to decreased light intensity

The guanosine nucleotides ppGpp and pppGpp have been found in the photosynthetic bacterium $\text{Rhodopseudomonas spheroides}$ supporting a suggestion that these are ubiquitous compounds in bacteria. ppGpp levels undergo an abrupt rise coincident with the cessation of RNA accumulation which occurs in this bacterium following a down-shift in incident light intensity. An abrupt decrease in the rate of [³H] uridine uptake into acid-precipitable material characteristic of light shift-down is also coincident with the rise in ppGpp levels. A regulatory role for this nucleotide in $\text{R. spheroides}$ similar to that proposed in other bacteria is suggested.