Calcium-Enhanced Twitching Motility in Xylella fastidiosa Is Linked to a Single PilY1 Homolog
ABSTRACTThe plant-pathogenic bacteriumXylella fastidiosais restricted to the xylem vessel environment, where mineral nutrients are transported through the plant host; therefore, changes in the concentrations of these elements likely impact the growth and virulence of this bacterium. Twitching motility, dependent on type IV pili (TFP), is required for movement against the transpiration stream that results in basipetal colonization. We previously demonstrated that calcium (Ca) increases the motility ofX. fastidiosa, although the mechanism was unknown. PilY1 is a TFP structural protein recently shown to bind Ca and to regulate twitching and adhesion in bacterial pathogens of humans. Sequence analysis identified threepilY1homologs inX. fastidiosa(PD0023, PD0502, and PD1611), one of which (PD1611) contains a Ca-binding motif. Separate deletions of PD0023 and PD1611 resulted in mutants that still showed twitching motility and were not impaired in attachment or biofilm formation. However, the response of increased twitching at higher Ca concentrations was lost in thepilY1-1611 mutant. Ca does not modulate the expression of any of theX. fastidiosaPilY1 homologs, although it increases the expression of the retraction ATPasepilTduring active movement. The evidence presented here suggests functional differences between the PilY1 homologs, which may provideX. fastidiosawith an adaptive advantage in environments with high Ca concentrations, such as xylem sap.