Ngo, J.T. et al. Nat. Chem. Biol. 12, 459–465 (2016).

Correlative light and electron microscopy (CLEM) is a powerful tool for studying the precise organization of structures in cells. However, most labeling strategies for CLEM have focused on labeling proteins of interest, rather than nonprotein biomolecules. Ngo et al. addressed this gap by developing the Click-EM strategy for labeling nucleic acids, lipids and glycans. In Click-EM, metabolic analogs containing bio-orthogonal functional groups are incorporated into the target molecule by the cellular machinery. Singlet-oxygen-generating dyes, which can be used for fluorescence imaging, are then added to these analogs via click chemistry. These dyes are also capable of polymerizing diaminobenzidine (DAB) to generate contrast in electron microscopy for correlative imaging. The group used their labeling strategy to target DNA, RNA and lipids in mammalian cells and to label peptidoglycans in Gram-positive bacteria.