Getting to the point: unipolar growth of Hyphomicrobiales.

The governing principles and suites of genes for lateral elongation or incorporation of new cell wall material along the length of a rod-shaped cell are well described. In contrast, relatively little is known about unipolar elongation or incorporation of peptidoglycan at one end of the rod. Recent work in three related model systems of unipolar growth (Agrobacterium tumefaciens, Brucella abortus, and Sinorhizobium meliloti) has clearly established that unipolar growth in the Hyphomicrobiales order relies on a set of genes distinct from the canonical elongasome. Polar incorporation of envelope components relies on homologous proteins shared by the Hyphomicrobiales, reviewed here. Ongoing and future work will reveal how unipolar growth is integrated into the alphaproteobacterial cell cycle and coordinated with other processes such as chromosome segregation and cell division.

Lipopolysaccharide biosynthesis and traffic in the envelope of the pathogen Brucella abortus.

Lipopolysaccharide is essential for most Gram-negative bacteria as it is a main component of the outer membrane. In the pathogen Brucella abortus, smooth lipopolysaccharide containing the O-antigen is required for virulence. Being part of the Rhizobiales, Brucella spp. display unipolar growth and lipopolysaccharide was shown to be incorporated at the active growth sites, i.e. the new pole and the division site. By localizing proteins involved in the lipopolysaccharide transport across the cell envelope, from the inner to the outer membrane, we show that the lipopolysaccharide incorporation sites are determined by the inner membrane complex of the lipopolysaccharide transport system. Moreover, we identify the main O-antigen ligase of Brucella spp. involved in smooth lipopolysaccharide synthesis. Altogether, our data highlight a layer of spatiotemporal organization of the lipopolysaccharide biosynthesis pathway and identify an original class of bifunctional O-antigen ligases.