Regulated expression of AmyQ ␣-amylase of Bacillus amyloliquefaciens was used to examine the capacity of the protein secretion apparatus of B. subtilis. One B. subtilis cell was found to secrete maximally 10 fg of AmyQ per h. The signal peptidase SipT limits the rate of processing of the signal peptide. Another limit is set by PrsA lipoprotein. The wild-type level of PrsA was found to be 2 ؋ 10 4 molecules per cell. Decreasing the cellular level of PrsA did not decrease the capacity of the protein translocation or signal peptide processing steps but dramatically affected secretion in a posttranslocational step. There was a linear correlation between the number of cellular PrsA molecules and the number of secreted AmyQ molecules over a wide range of prsA and amyQ expression levels. Significantly, even when amyQ was expressed at low levels, overproduction of PrsA enhanced its secretion. The finding is consistent with a reversible interaction between PrsA and AmyQ. The high cellular level of PrsA suggests a chaperone-like function. PrsA was also found to be essential for the viability of B. subtilis. Drastic depletion of PrsA resulted in altered cellular morphology and ultimately in cell death.Proteins synthesized with a signal peptide are secreted from bacterial cells by the action of the protein secretion apparatus, which consists of several components involved in protein targeting, translocation, signal peptide processing, and posttranslocational folding (4, 7). Extensive studies of Escherichia coli and Bacillus subtilis have identified and characterized to a substantial extent the components of the apparatus that translocates secretory proteins across the cytoplasmic membrane. In B. subtilis, they include the SecY, SecE, and SecG proteins, which form the core of the translocation channel or translocator (30,47) and are associated in the membrane with the SecDF protein (3). Furthermore, there are several signal peptidases (43, 45). The role of SecA ATPase on the cis side of the membrane in targeting and coupling the energy required for translocation has been well established (15,48). Many components of the secretion apparatus are known to be under temporal control; their maximal level of expression parallels the onset of protein secretion in the early stationary growth phase (15, 43).The stages of protein secretion that take place outside the cytoplasmic membrane are less well understood. A central feature of secretion is posttranslocational folding. The correct folding of many secreted proteins is not spontaneous but dependent on assisting folding factors. In E. coli they include protein-specific chaperones, periplasmic peptidyl-prolyl cis/trans isomerases, and enzymes (Dsb proteins) involved in the formation and rearrangement of disulfide bonds (8,16,19,31,32). The depletion of foldases such as SurA and PpiD causes misfolding stress that activates the E -and cpx-dependent stress response (5,6,31). This results in the induction of expression of the periplasmic protease and foldases (6, 37), often resulting in t...