X-RAY CRYSTALLOGRAPHY SHOWS THAT TRANSLATIONAL INITIATION-FACTOR IF3 CONSISTS OF 2 COMPACT ALPHA BETA DOMAINS LINKED BY AN ALPHA-HELIX/

The structures of the two domains of translational initiation factor I F3 from Bacillus stearothermophilus have been solved by X-ray crystall ography using single wavelength anomalous scattering and multiwaveleng th anomalous diffraction, Each of the two domains has an alpha/beta to pology, with an exposed beta-sheet that is reminiscent of several ribo somal and other RNA binding proteins, An alpha-helix that protrudes ou t from the body of the N-terminal domain towards the C-terminal domain suggests that IF3 consists of two RNA binding domains connected by an alpha-helix and that it may bridge two regions of the ribosome. This represents the first high resolution structural information on a trans lational initiation factor.