YirB inhibits YjbH-enhanced proteolysis of Spx by ClpXP. (A) Spx (6 μM), ClpX (6 μM), and ClpP (12 μM) were incubated at 30°C in the presence of ATP, an ATP-generating system (creatine kinase), and 10 mM DTT with or without YjbH (6 μM) or YirB (0.5 μM). The top panel shows an SDS-PAGE gel profile of reactions without YjbH. The second panel shows a gel profile of reaction mixtures containing YjbH, and the bottom panel is a gel profile of reaction mixtures containing YirB. (B) A plot of Spx band intensities with the ClpP band as an internal control is shown as the standard error of the mean from three independent preparations.