Excretion of the egl gene product of Pseudomonas solanacearum

Abstract

Pseudomonas solanacearum is an important phytopathogen which excretes a variety of extracellular enzymes. Pulse-chase experiments showed that one of these enzymes, a beta-1,4-endoglucanase (EGL) encoded by the egl gene, is synthesized as a higher-molecular-weight precursor polypeptide (pEGL) which is subsequently excreted into the extracellular medium as a 43-kilodalton mature protein. S1 nuclease transcript mapping and DNA sequence analysis were used to identify the transcription start site and the possible translation start site of egl. Pulse-chase experiments and comparison of the putative NH2-terminal amino acid sequence of pEGL with the actual NH2-terminal amino acid sequence of mature excreted EGL suggested that pEGL has a 45-residue leader sequence preceding the N terminus of EGL which is proteolytically cleaved during export to the extracellular environment. The first 20 residues of the leader sequence resembled a typical lipoprotein signal peptide. The excretion of EGL by P. solanacearum apparently requires a membrane potential since it was blocked by carbonyl cyanide m-chlorophenyl hydrazone.