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Reversible Phase Variation in the phnE Gene, Which Is Required for Phosphonate Metabolism in Escherichia coli K-12. Samina Iqbal, 2004.It is known that Escherichia coli K-12 is cryptic [Phn–] for utilization of methyl phosphonate [MePn] and that Phn+ variantscan be selected for growth on MePn as the sole P source . Variantsarise from deletion via a possible slip strand mechanism of one of three direct 8-bp repeat sequences in phnE, which restores function to a component of a putative ABC type transporter. Here we show that Phn+ variants are present at the surprisingly high frequency of >10–2 in K-12 strains . Amplified-fragment length polymorphism analysis was used to monitor instabilityin phnE in various strains growing under different conditions. This revealed that, once selection for growth on MePn is removed, Phn+ revertants reappear and accumulate at high levels through reinsertion of the 8-bp repeat element sequence . It appears that, in K-12, phnE contains a high-frequency reversible gene switch, producing phase variation which either allows ["on"form] or blocks ["off" form] MePn utilization . The switch canalso block usage of other metabolizable alkyl phosphonates,including the naturally occurring 2-aminoethylphosphonate . AllK-12 strains, obtained from collections, appear in the "off"form even when bearing mutations in mutS, mutD, or dnaQ whichare known to enhance slip strand events between repetitive sequences.The ability to inactivate the phnE gene appears to be uniqueto K-12 strains since the B strain is naturally Phn+ and lacksthe inactivating 8-bp insertion in phnE, as do important pathogenic strains for which genome sequences are known and also strains isolated recently from environmental sources.
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