Journal of Bacteriology, November 1999, p. 6739-6746, Vol. 181, No. 21
ISSN 0021-9193
Copyright © 1999, American Society for Microbiology. All rights reserved.

Genetic and Structural Analyses of Cytoplasmic Filaments of Wild-Type Treponema phagedenis and
a Flagellar Filament-Deficient Mutant

Jacques Izard,* William A. Samsonoff, Mary Beth Kinoshita, and Ronald J. Limberger

David Axelrod Institute for Public Health, Wadsworth Center, New York State Department of Health, Albany,
New York 12201-2002

Unique cytoplasmic filaments are found in the treponeme genus of spirochete bacteria. Their function is unknown,
but their location underneath the periplasmic flagellar filaments (PFF) suggests a role in motility and/or cell structure.
To better understand these unique structures, the gene coding for the cytoplasmic filaments, cfpA, was identified in
various treponemal species. Treponema phagedenis cfpA was 2,037 nucleotides long, and the encoded polypeptide
showed 78 to 100% amino acid sequence identity with the partial 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 cytoplasmic filaments and the PFF. The number of cytoplasmic filaments per
cell of 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-to-one correlation at the cell end. Moreover, a structural link between these structures
could not be demonstrated. The cytoplasmic filaments were also analyzed by electron microscopy at different stages of
cell growth; this analysis revealed that they are cleaved before or during septum formation and before the nascent
formation of PFF. A PFF-deficient mutant of T. phagedenis possessed cytoplasmic filaments similar to those of
the wild type, suggesting 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 that the cytoplasmic filaments and the flagellar apparatus are physically linked.

* Corresponding author.

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