Complete functional analysis of type IV pilus components of a reemergent plant pathogen reveals neofunctionalization of paralog genes

Abstract

Type IV pilus (TFP) is a multifunctional bacterial structure involved in twitching motility, adhesion, biofilm formation, as well as natural competence. Here, by mutagenesis and functional analysis, we dissected the roles of all genes required for TFP biosynthesis and regulation in the reemergent plant pathogenic fastidious prokaryote Xylella fastidiosa. This xylem-limited, insect-transmitted pathogen lives constantly under flow conditions and therefore is highly dependent on TFP for host colonization. In addition, TFP-mediated natural transformation is a process that impacts genomic diversity and environmental fitness. Ten out of the thirty-eight genes analyzed were essential for movement and natural competence. Interestingly, seven sets of paralogs exist, and mutations showed opposing phenotypes, indicating evolutionary neofunctionalization of subunits within TFP. The minor pilin FimT3 was the only protein exclusively required for natural competence. We determined that FimT3 (but not the other two FimT paralogs) is a DNA receptor that is conserved among X. fastidiosa strains and binds DNA non-specifically via an electropositive surface. Among plant pathogens, this gene was also found in the genome of strains of the plant associated Xanthomonadaceae family. Overall, we highlight here the complex regulation of TFP in X. fastidiosa, providing a blueprint to understand TFP in other bacteria living under flow conditions.