The Effects of DNA-Binding Proteins on Insertion Sequence Element Transposition Upstream of the bgl Operon in Escherichia coli

The bglGFB operon in Escherichia coli K-12 strain BW25113, encoding the proteins necessary for the uptake and metabolism of β-glucosides, is normally not expressed. Insertion of either IS1 or IS5 upstream of the bgl promoter activates expression of the bgl operon when the cells are starving in the presence of a β-glucoside, drastically increasing transcription and allowing the cell to survive using this carbon source. Details surrounding the exact mechanism and regulation of the IS insertional event remain unclear. In this work, the role of several DNA-binding proteins influencing the rates of insertion upstream of bgl are examined via mutation assays and protocols measuring transcription. Both cAMP-activated Crp and IHF exert a positive effect on insertional Bgl+ mutations when present in the cell. The mechanism of how these two proteins cause this effect are discussed. Our results characterize IHF’s effect in conjunction with other mutations, show that IHF’s effect on IS insertion rate into bgl also affects other operons, and indicate that it likely exerts its effect by binding to and altering the DNA conformation of IS1 and IS5 in their native locations, rather than by directly influencing transposase gene expression. By contrast, the cyclic AMP-CRP complex acts directly on the bgl operon by binding to its site upstream of the bgl promoter