Bacteria cluster thousands of transmembrane chemoreceptors at opposite ends of the cell, allowing them to detect and follow food molecule gradients. Might the formation and maintenance of such clusters occur via stochastic assembly? To test this idea, Greenfield et at. used pho-toactivated localization microscopy (PALM) to count single fluorophore-tagged receptors with an optical resolution of 15 nm. They analyzed 1 million receptors and observed that many were present as singletons or small clusters in lateral regions of the cell (shown at left). A mathematical model in which the receptors are inserted randomly into the membrane, but can then be captured and incorporated into existing clusters, accounted for the observed distribution and predicted that the density of new clusters would be highest at a point farthest from a large cluster. Hence, through stochastic assembly, a cell with a large cluster at one pole will form a new large cluster at the opposite pole.
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