Trends Microbiol 2007 May 5;15(5):196-202. Epub 2007 Apr 5.
Laboratory of Sialobiology, Department of Pathobiology, University of Illinois at Urbana-Champaign, Urbana, IL 61802, USA.
Phase variation is usually thought of as the stochastic switching between alternatively expressed ('on') and unexpressed ('off') phenotypic states. However, coupling synthesis of a monotonous homopolysaccharide to a mechanism of random but incomplete chemical modification produces almost infinite structural variation. Potentially limitless variability implies that evolution can produce highly ornate or extravagant flourishes reminiscent of the baroque style. Here, we describe an analysis of capsular polysialic acid form variation in Escherichia coli K1, demonstrating that the large number of variant structures is controlled by a single contingency locus. The mechanism for generating maximum structural diversity from maximal genetic parsimony is conferred by a simple translational switch carried on a K1-specific prophage.