Three-dimensional structure of the E. coli DMA-binding protein FIS

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Abstract
THE factor for inversion stimulation, FIS, is involved in several cellular processes, including site-specific recombination and tran-scriptional activation1–4. In the reactions catalysed by the DNA invertases Gin, Hin and Cin, FIS stimulates recombination by binding to an enhancer sequence1. Within the enhancer, two FIS dimers (each 2 x 98 amino acids)5–7 bind to two 15-base-pair consensus sequences8,9 (Fig. 1) and induce bending of DNA10,11. Current models propose that the enhancer–FIS complex organizes a specific synapse, either through direct interactions with Gin, or by modelling the substrate into a configuration suitable for recombination1,9,12. Using X-ray analysis at 2.0Å resolution, we now show that FIS is composed of four α helices tightly intertwined to form a globular dimer with two protruding helix–turn–helix motifs. The 24 N-terminal amino acids are so poorly defined in the electron density map as to make interpretation doubtful, indicating that they might act as 'feelers' suitable for DNA or protein (invertase) recognition. We infer from model building that DNA has to bend for tight binding to FIS.