Characterization of zoospore type IV pili in Actinoplanes missouriensis

The rare actinomycete Actinoplanes missouriensis produces terminal sporangia containing a few hundred flagellated spores. After release from the sporangia, the spores swim rapidly in aquatic environments as zoospores. The zoospores stop swimming and begin to germinate in niches for vegetative growth. Here, we report the characterization and functional analysis of zoospore type IV pili in A. missouriensis . The pilus gene ( pil ) cluster, consisting of three apparently σ FliA -dependent transcriptional units, is activated during sporangium formation similar to the flagellar gene cluster, indicating that the zoospore has not only flagella but also pili. With a new method in which zoospores were fixed with glutaraldehyde to prevent pilus retraction, zoospore pili were observed relatively easily using transmission electron microscopy: 6 ± 3 pili per zoospore (n = 37 piliated zoospores) and 0.62 ± 0.35 μm in length (n = 206), via observation of fliC -deleted, non-flagellated zoospores. No pili were observed in the zoospores of a prepilin-encoding pilA deletion (Δ pilA ) mutant. In addition, the deletion of pilT , which encodes an ATPase predicted to be involved in pilus retraction, substantially reduced the frequency of pilus retraction. Several adhesion experiments using wild-type and Δ pilA zoospores indicated that the zoospore pili are required for the sufficient adhesion of zoospores to hydrophobic solid surfaces. Many zoospore-forming rare actinomycetes conserve the pil cluster, which indicates that the zoospore pili yield an evolutionary benefit in the adhesion of zoospores to hydrophobic materials as footholds for germination in their mycelial growth. IMPORTANCE Bacterial zoospores are interesting cells in that their physiological state changes dynamically: they are dormant in sporangia, exert temporary mobility after awakening, and finally stop swimming to germinate in niches for vegetative growth. However, the cellular biology of a zoospore remains largely unknown. This study describes unprecedented zoospore type IV pili in the rare actinomycete Actinoplanes missouriensis . Similar to usual bacterial type IV pili, zoospore pili appeared to be retractable. Our findings that the zoospore pili have a functional role in the adhesion of zoospores to hydrophobic solid surfaces and that the zoospores use both pili and flagella properly according to their different purposes provide an important insight into the cellular biology of the zoospore.