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

Abstract

THE factor for inversion stimulation, FIS, is involved in several cellular processes, including site-specific recombination and tran-scriptional activation^1–4. 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 sequences^8,9 (Fig. 1) and induce bending of DNA^10,11. 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 recombination^1,9,12. 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.