Complete loss of lipoprotein biosynthesis machinery in an obligate intracellular bacterium
Orientia tsutsugamushi is a cytoplasm-dwelling, obligate intracellular, diderm bacterium with an unusual cell envelope. It lacks lipopolysaccharide and possesses only a minimal peptidoglycan-like cell wall. Here, we report that O. tsutsugamushi also lacks the two acyltransferase components of the lipoprotein biosynthesis pathway (Lgt, Lnt) and the entire lipoprotein transport machinery (Lol system). O. tsutsugamushi encodes several proteins that are conserved as lipoproteins among other organisms, including Pal and components of the beta-barrel assembly machine (BAM) complex, but these lack the canonical lipobox motif, consistent with the absence of lipidation pathways. Instead, we find that O. tsutsugamushi Pal compensates for the lack of lipidation by incorporating two predicted amphipathic helices, a feature not observed in Pal homologues from other organisms. These findings suggest that O. tsutsugamushi has compensated for a lack of lipoprotein assembly machinery through alternative mechanisms. We speculate on the possible evolutionary advantage of this highly unusual loss of the lipoprotein biosynthesis machinery in the context of an already minimal cell envelope.