10.5 Targeting


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    Unlike your cell, phage cannot direct their movement. They simply ride the currents of Brownian motion, waiting to bump into a target receptor they can latch onto. Their abundance helps; in the ocean, there are about ten times as many phage as bacteria. Still, it might be a long wait, so evolution has fashioned some mechanisms to speed things up. One strategy is to target a receptor not on the cell body itself, but on a long, more easily-encountered cell appendage.

    Some species of bacteria, including this Escherichia coli use “competence pili” assembled by a type IV secretion system to take up DNA from other cells. This “horizontal gene transfer” (in contrast to “vertical” heredity from mother to daughter) lets asexually-reproducing cells mix and match genes to potential advantage. It also blurs the line between species (see the Phylogenetic Tree for more about that). And in this case, it offers an advantage to these MS2 leviphage, which have evolved to recognize the pilus and inject their own RNA into the nucleic acid uptake channel.