In gram-negative bacteria, the extracellular proteins have two lipid bilayers to cross, in contrast to the proteins targeted to the periplasm or the outer membrane. Depending upon the secretion route and the genes involved, two major secretion pathways can be identified. Without traversing the periplasm, oc-hemolysin of Escherichia coli is secreted through a hemolysin translocator composed of HlyB, HlyD (24), and ToiC (48). Two genes, prtD and prtE, required for the secretion of an extracellular protease by Erwinia chrysanthemi were found to be homologous to hlyB and hlyD, respectively (27). In the second category, extracellular proteins, for example, the pullulanase of Klebsiella oxytoca, are secreted via the periplasm (34). A closely linked operon includes at least 12 pul genes encoding proteins required for the secretion of pullulanase in addition to the sec genes involved in the translocation of proteins across the inner membrane (34, 35). Homologs of the pul genes are also required for the secretion of extracellular proteins by E. chrysanthemi (22), Pseudomonas aeruginosa (15), Xanthomonas campestns pv. campestris (12), and Yersinia enterocolitica (29). Mutations in these genes can result in the accumulation of extracellular proteins in the periplasm (1,7,11,16,29,34,45,49