Genetic and structural analyses of cytoplasmic filaments of wild-type Treponema phagedenis and a flagellar filament-deficient mutant

Unique cytoplasmic filaments are found in the treponeme genus of spirochete bacteria. Their function is unknown, but their location underneath the per iplasmic flagellar filaments (PFF) suggests a role in motility and/or cell structure. To better understand these unique structures, the gene coding fo r the cytoplasmic filaments, cfpA, was identified in various treponemal spe cies. Treponema phagedenis cfpA was 2,037 nucleotides long, and the encoded polypeptide showed 78 to 100% amino acid sequence identity with the partia l sequence of CfpA from T. denticola, T. vincentii, and T. pallidum subsp, pertenue, Wild-type T. phagedenis and a PFF-deficient isolate were analyzed by electron microscopy to assess the structural relationship of the cytopl asmic filaments and the PFF, The number of cytoplasmic filaments per cell o f T. phagedenis (mean, 5.7) was compared with the number of PFF at each end of the cell (mean, 4.7); the results suggest that there is no direct one-t o-one correlation at the cell end. Moreover, a structural link between thes e structures could not be demonstrated. The cytoplasmic filaments were also analyzed by electron microscopy at different stages of cell growth; this a nalysis revealed that they are cleaved before or during septum formation an d before the nascent formation of PFF, A PFF-deficient mutant of T. phagede nis possessed cytoplasmic filaments similar to those of the wild type, sugg esting that intact PFF are not required for their assembly and regulation. The extensive conservation of CfpA among pathogenic spirochetes suggests an important function, and structural analysis suggests that it is unlikely t hat the cytoplasmic filaments and the flagellar apparatus are physically li nked.