Chapter 17
- The eukaryotic initiation factor eIF5B is homologous to bacterial IF 2, and a similar factor in archaea. This universal factor plays one of the same roles in eukaryotes as IF 2 does in bacteria: facilitating ribosomal subunit joining. Roll-Mecak et al. have performed x-ray crystallography on three forms of an archaeal version of this protein, IF2/eIF5B, from Methanobacterium thermoautotrophicum. By comparing the structures of the free, GDP-bound and GTP-bound forms of the protein they found that GTP-binding causes a jointed lever to move and thereby transmit the effect of GTP-binding to the C-terminus of the protein, over 90 from the GTP-binding site. (Roll-Mecak, A, C. Cao, T.E. Dever, and S.K. Burley. 2000. X-ray structures of the universal translation initiation factor IF2/eIF5B: Conformational changes on GDP and GTP binding. Cell 103:781-92)
- One of the activities of bacterial initiation factor IF1 is to block the A site of the ribosome and prevent aminoacyl-tRNA binding there. Carter et al. have determined the crystal structure of the 30S ribosomal subunit of Thermus thermophilus, bound to IF1. They find that IF1 covers the A site and provides pockets that bury the important bases A1492 and A1493 of the 16S rRNA. This explains how IF1 prevents tRNA from binding to the A site. (Carter, A.P., W.M. Clemons Jr., D.E. Brodersen, R.J. Morgan-Warren, T. Hartsch, B.T. Wimberly, and V. Ramakrishnan. 2001. Crystal structure of an initiation factor bound to the 30S ribosomal subunit. Science 291:498-501)
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