Control of protein synthesis in Escherichia coli: Lack of correlation with changes in intracellular pools of ATP, GTP, and ppGpp

Abstract

Intracellular pools of ATP, GTP, and ppGpp have been measured in Escherichia coli after an energy source shift-down from glucose minimal to succinate-minimal medium. In a Tic + strain (ATCC 10798), which reduces translational initiation after the down-shift, the rate of protein labeling falls to about 30% of its preshift rate within the first minute after shift and reaches a minimum of 17% by 6 min after shift. The ATP pool in this strain remains constant for about 10 min after shift, then declines gradually to about 60% of its initial level. The temporal discrepancy between protein synthesis and the decline in the ATP pool indicates that a decrease in intracellular ATP is not necessary for the control of protein synthesis. In a Tic − strain (W1), which cannot control translational initiation under these conditions, the decline in the ATP pool is somewhat more rapid and more pronounced (to 40%) than in the Tic + strain, indicating that the decline in the ATP pool is not sufficient to trigger control of translational initiation. The intracellular GTP pool in the Tic + strain remains constant for 2 min after shift, then declines gradually to reach a minimum of 45% of its initial level at 20 min after shift. The pattern is in general similar in the the Tic − strain, although the ultimate decline in GTP is more pronounced (to 29%). These data indicate that the decline in GTP is not sufficient and probably unnecessary to elicit control of translational initiation. Intracellular levels of ppGpp increase with very similar kinetics in relA +Tic + (ATCC 10798) and relA +Tic − (W1) strains, indicating that elevated ppGpp levels are not sufficient to elicit control of translation. In a relA −Tic + strain (NF162), or in a relA +Tic + strain treated with rifampin, the ppGpp pool does not increase significantly after shift-down although translational initiation is reduced. Thus, an increase in the ppGpp pool is not necessary to control of translational initiation.