Two nonredundant SecA homologues function in mycobacteria

The proper extracytoplasmic localization of proteins is an important aspect of mycobacterial physiology and the pathogenesis of Mycobacterium tubercul osis. The protein export systems of mycobacteria have remained unexplored. The Sec-dependent protein export pathway has been well characterized in Esc herichia coli and is responsible for transport across the cytoplasmic membr ane of proteins containing signal sequences at their amino termini. SecA is a central component of this pathway, and it is highly conserved throughout bacteria. Here we report on an unusual property of mycobacterial protein e xport-the presence of two homologues of SecA (SecA1 and SecA2). Using an al lelic-exchange strategy in Mycobacterium smegmatis, we demonstrate that sec A1 is an essential gene. In contrast, secA2 can be deleted and is the first example of a nonessential secA homologue. The essential nature of secA1, w hich is consistent with the conserved Sec pathway, leads us to believe that secA1 represents the equivalent of E. coli secA. The results of a phenotyp ic analysis of a Delta secA2 mutant of M. smeginatis are presented here and also indicate a role for SecA2 in protein export. Based on our study, it a ppears that SecA2 can assist SecA1 in the export of some proteins via the S ee pathway. However, SecA2 is not the functional equivalent of SecA1. This finding, in combination with the fact that SecA2 is highly conserved throug hout mycobacteria, suggests a second role for SecA2. The possibility exists that another role for SecA2 is to export a specific subset of proteins.