Secretion Across the Cytoplasmic Membrane

All three L. pneumophila strains possess two systems for protein transport across the cytoplasmic membrane. The sec apparatus, which allows protein translocation in an unfolded state, is complete with the exception of secG. In addition, L. pneu-mophila encodes a twin arginine translocation (TAT) pathway for translocation of folded proteins. In contrast to the usual organization of tatA, tatB, and tatC in an operon, only the tatA and tatB genes are cotranscribed, and tatC is in a different location on the L. pneumophila chromosome. This organization is conserved in all three genomes, and similarity of the Tat proteins is between 97% and 100%. In strains Paris, Lens, and Philadelphia 1, the tatAB operon and the tatC gene are located 24 kbp, 41 kbp, and 27 kbp apart, respectively. However, the flanking regions are, despite the different distances between the tatAB and tatC genes, homologous in the three strains. The Tat secretion system is implicated in growth under low-iron conditions, and in growth within macrophages in an Lsp-indepen-dent manner (Rossier and Cianciotto 2005).

Tat substrates are characterized by two arginine residues in their signal peptide. Based on this criterion, 20 proteins were identified as putative Tat substrates from analysis of genome sequence, including 12 transport/binding proteins and lipoproteins and three proteins involved in respiration (De Buck et al. 2004). Secretion of a subunit of cytochrome c oxidase and of phospholipase C via this pathway has been confirmed (Rossier and Cianciotto 2005).

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