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Microbial Models II: Transposons and Regulation of Gene Expression 12 November, 2003 Text Chapter 18 Transposable elements (transposons) can move from one location to another in the genome. Insertion sequences, the simplest transposons, contain only a transposase gene flanked by two inverted repeats. In transposition, transposase cuts DNA at the target site. Then, it catalyzes the movement of the transposon to the new site. This movement can cause mutations in it moves the transposon into the coding sequence or regulatory regions of a gene. Composite transposons can carry genes from location to location if the genes are located between two insertion sequences. Regulation of Gene Expression Biochemical pathways are often regulated by feedback inhibition. Usually, the synthesis of the enzymes in the pathway is also regulated. This regulation is at the level of transcription. The trp operon is a biosynthetic operon - it codes for the enzymes that make the amino acid tryptophan. This pathway should be kept in the “off” state when tryptophan is present. When tryptophan is absent, the bacteria need to make it from scratch. Now the pathway needs to be turned on. The lac operon is a catabolic operon - it codes for the enzymes that burn the sugar lactose for fuel. This pathway should be kept in the “off” state when lactose is absent. When lactose is present, the bacteria can burn it for fuel. Now the pathway needs to be turned on. Furthermore, the bacterium does not burn lactose for fuel if its preferred carbon source (glucose) is available. The decision about the presence or absence of glucose is independent of the decision about the presence or absence of lactose, and is made by the CAP protein and the nucleotide cAMP. The cell only expresses the lac operon when lactose is present and glucose is absent.