Erwinia carotovora subsp. carotovora strain Ecc71 produces an array of extracellular enzymes including pectate lyase (Pel), polygalacturonase, cellulase, and protease. In strain Ecc71, these enzymes are coregulated by aepA, which encodes an activator of extracellular protein production (H. Murata, J. L. McEvoy, A. Chatterjee, A. Collmer, and A. K. Chatterjee, Mol. Plant-Microbe Interact, 4:239-246, 1991). The nucleotide sequence of a 2.7-kb aepA+ DNA segment revealed an open reading frame (ORF) of 1,395 bp which matches with the size of the aepA transcript determined by Northern blot analysis. aepA is predicted to encode a protein of 465 amino acid residues with a molecular mass of approximately 51 kDa and a pI of 6.52. The occurrence of a putative signal sequence and several hydrophobic domains suggest membrane localization of AepA. An aepA-lacZ operon fusion was constitutively expressed in E. coli (DH5 alpha) but inducible by pectate and celery extract in E. c. subsp. carotovora (AC5006). These findings suggest that aepA expression may be negatively regulated in E. c. subsp. carotovora. By assaying for the transcript of pel-1, which species a major secreted Pel species in strain Ecc71, and by following the expression of a pel1-lacZ operon fusion we determined that AepA activates pel-1 transcription. The characteristics of aepA including the lack of homology with other prokaryotic regulatory genes indicate that aepA encodes a novel regulatory protein required for extracellular protein production. Whereas homologs of Ecc71 aepA occur in E. c. subsp. carotovora and E. c. subsp. atroseptica strains, activation of exoenzyme production is markedly stimulated by aepA in E. c. subsp. carotovora than in E. c. subsp. atroseptica.