= FRET proximity readout experiment
{c}
In <Receptor sensitivity in bacterial chemotaxis by Victor Sourjik and others>, <Victor Sourjik> and <Howard C. Berg> used a cyan fluorescent tag on <CheZ> as <FRET donor> and a yellow tag on <CheY> as <FRET acceptor>. <Escherichia coli> <biological cells> adhered to a polylysine-coated coverslip in a continuously perfused chamber. A <fluorescence microscope> excited them with a $442$-nanometre <laser>; split emission bands fed two <photon>-counting detectors. The yellow/cyan emission <ratio> fell when attractant was added and rose when it was removed.<Receptor sensitivity in bacterial chemotaxis by Victor Sourjik and others>[Introduction, Materials and Methods: Preparation of Cells and Data Acquisition; Results and Figure 1]{ref}
Saturating attractant increased cyan emission by an amount matching selective bleaching of the yellow tag: removing the <FRET acceptor> supported relief of <FRET donor> transfer. Strains carrying the tagged pair but lacking other <chemotaxis> machinery showed no stimulus-dependent emission <ratio> change. Weak illumination reduced bleaching drift.<Receptor sensitivity in bacterial chemotaxis by Victor Sourjik and others>[Data Analysis and Results: acceptor bleaching comparison, low illumination, and flhC negative control]{ref}
The inference is a change in the population of nearby tagged <CheY>/<CheZ> pairs associated with <CheY> <phosphorylation>. It neither measures each pair's separation nor directly observes motor motion. The readout also averages many <biological cells>, and perfusion takes finite time. The pathway interpretation belongs to the <chemotaxis FRET assay>; the general measurement lesson is that molecular association can alter <FRET> while the geometry of an individual associated pair stays the same.
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