Abstract
The stringent response is important for bacterial survival under stressful conditions, such as amino acid starvation, and is characterized by the accumulation of ppGpp and pppGpp. ObgE (CgtA, YhbZ) is an essential conserved GTPase in
Escherichia coli
and several observations have implicated the protein in the control of the stringent response. However, consequences of the protein on specific responses to amino acid starvation have not been noted. We show that ObgE binds to ppGpp with biologically relevant affinity
in vitro
, implicating ppGpp as an
in vivo
ligand of ObgE. ObgE mutants increase the ratio of pppGpp to ppGpp within the cell during the stringent response. These changes are correlated with a delayed inhibition of DNA replication by the stringent response, delayed resumption of DNA replication after release, as well as a decreased survival to the response to amino acid deprivation. With this data, we place ObgE as an active effector of the response to amino acid starvation
in vivo
. Our data correlate the pppGpp/ppGpp ratio with DNA replication control under bacterial starvation conditions suggesting a possible role for the relative balance of these two nucleotides.