A RARE 920-KILOBASE CHROMOSOMAL INVERSION MEDIATED BY IS1 TRANSPOSITION CAUSES CONSTITUTIVE EXPRESSION OF THE YIAK-S OPERON FOR CARBOHYDRATE UTILIZATION IN ESCHERICHIA-COLI

The regulator of the yiaK-S operon, currently assigned a carbohydrate utilization function in Escherichia coli, is inactivated by a genome r earrangement that: leads to the constitutive expression of the operon, The yiaK-S constitutive cells acquire the ability to utilize the rare pentose L-lyxose. Restriction analysis and sequencing of the regulato r gene indicate that it is disrupted by foreign DNA. The insert consis ts of a larger, inverted fragment of DNA of 920 kilobases flanked by t wo ISI elements with opposite polarity. One corresponds to that found naturally at min 0.4 of the bacterial chromosome and tile other to a n ew copy transposed into the regulator gene located at min 80.6. This i nsertion-inversion could be the result of the intramolecular transposi tion mechanism itself, a gene rearrangement rarely originated by IS1, Alternatively, it could be attributed to the homologous recombination between the IS1 at min 0.4 and the IS1 transposed intermolecularly int o the yiaK-S regulator gene. The participation of a rare IS1-mediated inversion in the evolution of a stable phenotype is thus identified.