Multiple Copies of flhDC in Paraburkholderia unamae Regulate Flagellar Gene Expression, Motility, and Biofilm Formation.

FlhDC is a heterohexameric complex that acts as a master regulator of flagellar biosynthesis genes in numerous bacteria. Previous studies have identified a single flhDC operon encoding this complex. However, we found that two flhDC loci are present throughout Paraburkholderia, and two additional flhC copies are also present in Paraburkholderia unamae. Systematic deletion analysis in P. unamae of the different flhDC copies showed that one of the operons, flhDC1, plays the predominant role, with deletion of its genes resulting in a severe inhibition of motility and biofilm formation. Expression analysis using promoter-lacZ fusions and real-time quantitative PCR support the primary role of flhDC1 in flagellar gene regulation, with flhDC2 a secondary contributor. Phylogenetic analysis shows the presence of the flhDC1 and flhDC2 operons throughout Paraburkholderia. In contrast, Burkholderia and other bacteria only carry the copy syntenous with flhDC2. The variations in impact each copy of flhDC has on downstream processes indicate that regulation of FlhDC in P. unamae, and likely other Paraburkholderia species, is regulated at least in part by the presence of multiple copies of these genes. IMPORTANCE Motility is important in the colonization of plant roots by beneficial and pathogenic bacteria, with flagella playing essential roles in host cell adhesion, entrance, and biofilm formation. Flagellar biosynthesis is energetically expensive. Its complex regulation by the FlhDC master regulator is well studied in peritrichous flagella expressing enterics. We report the unique presence throughout Paraburkholderia of multiple copies of flhDC. In P. unamae, the flhDC1 copy showed higher expression and a greater effect on swim motility, flagellar development, and regulation of downstream genes, than the flhDC2 copy that is syntenous to flhDC in Escherichia coli and pathogenic Burkholderia spp. The flhDC genes have evolved differently in these plant-growth-promoting bacteria, giving an additional layer of complexity in gene regulation by FlhDC.

Structure of the apo anti-influenza CH65 Fab.

Influenza viruses remain a persistent challenge to human health owing to their inherent ability to evade the immune response by antigenic drift. However, the discovery of broadly neutralizing antibodies (bnAbs) against divergent viruses has sparked renewed interest in a universal influenza vaccine and novel therapeutic opportunities. Here, a crystal structure at 1.70 Å resolution is presented of the Fab of the human antibody CH65, which has broad neutralizing activity against a range of seasonal H1 isolates. Previous studies proposed that affinity maturation of this antibody lineage pre-organizes the complementarity-determining region (CDR) loops into an energetically favorable HA-bound conformation. Indeed, from the structural comparisons of free and HA-bound CH65 presented here, the CDR loops, and in particular the heavy-chain CDR3, adopt the same conformations in the free and bound forms. Thus, these findings support the notion that affinity maturation of the CH65 lineage favorably preconfigures the CDR loops for high-affinity binding to influenza hemagglutinin.