LpqM, a Mycobacterial Lipoprotein-Metalloproteinase, Is Required for Conjugal DNA Transfer in Mycobacterium smegmatis

We have previously described a novel conjugal DNA transfer process that occurs in Mycobacterium smegmatis. To identify donor genes required for transfer, we have performed a transposon mutagenesis screen; we report here that LpqM, a putative lipoprotein-metalloproteinase, is essential for efficient DNA transfer. Bioinformatic analyses predict that LpqM contains a signal peptide necessary for the protein's targeting to the cell envelope and a metal ion binding motif, the likely catalytic site for protease activity. Using targeted mutagenesis, we demonstrate that each of these motifs is necessary for DNA transfer and that LpqM is located in the cell envelope. The requirement for transfer is specific to the donor strain; an lpqM knockout mutant in the recipient is still proficient in transfer assays. The activity of LpqM is conserved among mycobacteria; homologues from both Mycobacterium tuberculosis and Mycobacterium avium can complement lpqM donor mutants, suggesting that the homologues recognize and process similar proteins. Lipoproteins constitute a significant proportion of the mycobacterial cell wall, but despite their abundance, very few have been assigned an activity. We discuss the potential role of LpqM in DNA transfer and the implications of the conservation of LpqM activity in M. tuberculosis.