To assess the utility of the cSeNP for imparting contrast to proteins in cellular electron microscopy, we genetically fused cSeNP DNA to DNA encoding the 'filamentous temperature sensitive protein Z (FtsZ), a bacterial tubulin homologue. FtsZ, along with at least 13 other division proteins, aids in cleaving a parent cell into daughter cells during cell division. FtsZ forms protofilaments observed in two forms. In non-dividing cells, FtsZ is found in membrane associated dispersed helical protofilaments, comprised of 30 -80 FtsZ protomers. During cell division, FtsZ protofilaments localize to the midpoint (cleavage furrow) of the dividing cell, forming a so-called Z ring 95,99 (Figure , panel A). Protofilaments on average lie 16 nm from the cellular inner membrane. Protofilaments can extend to lengths of 100 nm. We hypothesized that expression of cSeNP-FtsZ in E. coli would result in SeNP/filament conjugates, with SeNPs organized by FtsZ at different points in the cell cycle as shown in Figure , panel B. We expressed a c-terminal FtsZ-cSeNP fusion protein in E. coli BL21 cells. The population of FtsZ protomers inside the cell, therefore, is comprised of both genomic FtsZ and recombinant plasmid-expressed FtsZ-cSeNP. FtsZ overexpression interferes with completion of cell division. This results in elongated cells extending up to 20 -30 µm in length. A normal cell is ca. 3 -5 µm). In such extended cells observations of helical protofilaments forming wave-like patterns along the membrane are known. Technical details of cSeNP-FtsZ DNA construction and expression are found in methods (M1) Substantial optimization work (described in SI-cSe_NP_Expression_Optimization) was performed with the highest-throughput method of cellular preservation for EM: drop casting of glutaraldehyde-fixed cells on TEM grids (Methods, M2). In these experiments, we identified conditions where FtsZ-cSeNP resulted in Se and SeNP distributions consistent with expected FtsZ locations (Figure , panels A and B). Figure , panel C shows cells grown in 2 mM SeO32-expressing the cSeNP-FtsZ construct under 100 uM IPTG; Figure , panel D shows cells prepared the same way except without SeO3 2-. In panel C, when SeO3 2-is present, contrast increases at the cleavage furrow, where FtsZ-cSeNP concentration is expected to be high. In addition, putative longitudinal FtsZ-cSeNP filaments are observed, consistent with prior observations of FtsZ overexpression by others. Figure , panel D shows control cells grown without SeO32-. Contrast appears uniform across these cells. Figure , panel E shows elemental mapping of Se in a dividing cell collected by scanning transmission electron microscopy-electron X-ray dispersive spectroscopy (STEM-EDS), confirming that the densities at the cleavage furrow are Se-rich.