Negative-staining EM allowed purity and concentration assessment, as well as adapting the T5 tail:FhuA ratio prior to cryo-grid preparation. Negative-stain grids of all samples were prepared using the mica-carbon flotation technique over a 2% sodium silicotungstate solution and were observed with an FEI/Thermo Fisher Scientific Tecnai 12 microscope (LaB6 electron source, 120 kV) equipped with a Gatan Orius 1000 CCD camera.
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