Eukaryotic and archaeal translation initiation factor 2: A heterotrimeric tRNA carrier

FEBS Letters

Volumn 584, Issue 2, 2010, Pages 405-412

  Schmitt, Emmanuelle ^a   Naveau, Marie ^a   Mechulam, Yves ^a  

^a LABORATOIRE DE BIOCHIMIE   (France)

Author keywords

G protein; Initiator tRNA; Translation initiation

Indexed keywords

GUANINE NUCLEOTIDE; HETERODIMER; HETEROTRIMERIC GUANINE NUCLEOTIDE BINDING PROTEIN; INITIATION FACTOR 2ALPHA; MESSENGER RNA; PROTEIN SUBUNIT; TRANSFER RNA;

ALPHA CHAIN; BETA CHAIN; CONTROLLED STUDY; NONHUMAN; PRIORITY JOURNAL; PROTEIN ANALYSIS; PROTEIN CONFORMATION; PROTEIN FUNCTION; PROTEIN RNA BINDING; PROTEIN STRUCTURE; REVIEW; START CODON; STRUCTURE ANALYSIS; TRANSLATION INITIATION; TRANSLATION REGULATION;

ARCHAEA; EUKARYOTA; EUKARYOTIC INITIATION FACTOR-2; PROTEIN CONFORMATION; PROTEIN MULTIMERIZATION; PROTEIN SUBUNITS; RNA, TRANSFER;

ARCHAEA; EUKARYOTA;

EID: 71849102917     PISSN: 00145793     EISSN: None     Source Type: Journal    
DOI: 10.1016/j.febslet.2009.11.002     Document Type: Review

References (102)

1. Levin D.H., Kyner D., and Acs G. Protein initiation in eukaryotes: formation and function of a ternary complex composed of a partially purified ribosomal factor, methionyl transfer RNA, and guanosine triphosphate. Proc. Natl. Acad. Sci. USA 70 (1973) 41-45
2. Schreier M.H., and Staehelin T. Initiation of eukaryotic protein synthesis: (Met-tRNAf-40S ribosome) initiation complex catalysed by purified initiation factors in the absence of mRNA. Nat. New Biol. 242 (1973) 35-38
3. Safer B., Anderson W.F., and Merrick W.C. Purification and physical properties of homogeneous initiation factor MP from rabbit reticulocytes. J. Biol. Chem. 250 (1975) 9067-9075
4. Safer B., Adams S.L., Anderson W.F., and Merrick W.C. Binding of Met-tRNAf and GTP to homogeneous initiation factor MP. J. Biol. Chem. 250 (1975) 9076-9082
5. Barrieux A., and Rosenfeld M.G. Characterization of GTP-dependent Met-tRNAf binding protein. J. Biol. Chem. 252 (1977) 3843-3847
6. Picciano D.J., Prichard P.M., Merrick W.C., Shafritz D.A., Graf H., Crystal R.G., and Anderson W.F. Isolation of protein synthesis initiation factors from rabbit liver. J. Biol. Chem. 248 (1973) 204-214
7. Lloyd M.A., Osborne Jr. J.C., Safer B., Powell G.M., and Merrick W.C. Characteristics of eukaryotic initiation factor 2 and its subunits. J. Biol. Chem. 255 (1980) 1189-1193
8. Cigan A.M., Pabich E.K., Feng L., and Donahue T.F. Yeast translation initiation suppressor sui2 encodes the alpha subunit of eukaryotic initiation factor 2 and shares sequence identity with the human alpha subunit. Proc. Natl. Acad. Sci. USA 86 (1989) 2784-2788
9. Pathak V.K., Nielsen P.J., Trachsel H., and Hershey J.W. Structure of the beta subunit of translational initiation factor eIF-2. Cell 54 (1988) 633-639
10. Donahue T.F., Cigan A.M., Pabich E.K., and Valavicius B.C. Mutations at a Zn(II) finger motif in the yeast eIF-2 beta gene alter ribosomal start-site selection during the scanning process. Cell 54 (1988) 621-632
11. Hannig E.M., Cigan A.M., Freeman B.A., and Kinzy T.G. GCD11, a negative regulator of GCN4 expression, encodes the gamma subunit of eIF-2 in Saccharomyces cerevisiae. Mol. Cell. Biol. 13 (1993) 506-520
12. Cigan A.M., Feng L., and Donahue T.F. tRNAi(met) functions in directing the scanning ribosome to the start site of translation. Science 242 (1988) 93-97
13. Harashima S., and Hinnebusch A.G. Multiple GCD genes required for repression of GCN4, a transcriptional activator of amino acid biosynthetic genes in Saccharomyces cerevisiae. Mol. Cell. Biol. 6 (1986) 3990-3998
14. Dorris D.R., Erickson F.L., and Hannig E.M. Mutations in GCD11, the structural gene for eIF-2 gamma in yeast, alter translational regulation of GCN4 and the selection of the start site for protein synthesis. EMBO J. 14 (1995) 2239-2249
15. Huang H.K., Yoon H., Hannig E.M., and Donahue T.F. GTP hydrolysis controls stringent selection of the AUG start codon during translation initiation in Saccharomyces cerevisiae. Genes Dev. 11 (1997) 2396-2413
16. Unbehaun A., Borukhov S.I., Hellen C.U., and Pestova T.V. Release of initiation factors from 48S complexes during ribosomal subunit joining and the link between establishment of codon-anticodon base-pairing and hydrolysis of eIF2-bound GTP. Genes Dev. 18 (2004) 3078-3093
17. Algire M.A., Maag D., and Lorsch J.R. Pi release from eIF2, not GTP hydrolysis, is the step controlled by start-site selection during eukaryotic translation initiation. Mol. Cell 20 (2005) 251-262
18. Majumdar R., and Maitra U. Regulation of GTP hydrolysis prior to ribosomal AUG selection during eukaryotic translation initiation. EMBO J. 24 (2005) 3737-3746
19. Cheung Y.N., et al. Dissociation of eIF1 from the 40S ribosomal subunit is a key step in start codon selection in vivo. Genes Dev. 21 (2007) 1217-1230
20. Dennis P.P. Ancient ciphers: translation in archaea. Cell 89 (1997) 1007-1010
21. Kyrpides N.C., and Woese C.R. Archaeal translation initiation revisited: the initiation factor 2 and eukaryotic initiation factor 2B alpha-beta-delta subunit families. Proc. Natl. Acad. Sci. USA 95 (1998) 3726-3730
22. Dhaliwal S., and Hoffman D.W. The crystal structure of the N-terminal region of the alpha subunit of translation initiation factor 2 (eIF2alpha) from Saccharomyces cerevisiae provides a view of the loop containing serine 51, the target of the eIF2alpha-specific kinases. J. Mol. Biol. 334 (2003) 187-195
23. Ito T., Marintchev A., and Wagner G. Solution structure of human initiation factor eIF2alpha reveals homology to the elongation factor eEF1B. Struct. Fold. Des. 12 (2004) 1693-1704
24. Nonato M.C., Widom J., and Clardy J. Crystal structure of the N-terminal segment of human eukaryotic translation initiation factor 2alpha. J. Biol. Chem. 277 (2002) 17057-17061
25. Yatime L., Mechulam Y., Blanquet S., and Schmitt E. Structural switch of the gamma subunit in an archaeal aIF2 alpha gamma heterodimer. Structure 14 (2006) 119-128
26. Yatime L., Schmitt E., Blanquet S., and Mechulam Y. Structure-function relationships of the intact aIF2α subunit from the archaeon Pyrococcus abyssi. Biochemistry 44 (2005) 8749-8756
27. Tahara M., Ohsawa A., Saito S., and Kimura M. In vitro phosphorylation of initiation factor 2 alpha (aIF2 alpha) from hyperthermophilic archaeon Pyrococcus horikoshii OT3. J. Biochem. 135 (2004) 479-485
28. Das S., Maiti T., Das K., and Maitra U. Specific interaction of eukaryotic translation initiation factor 5 (eIF5) with the beta-subunit of eIF2. J. Biol. Chem. 272 (1997) 31712-31718
29. Asano K., Krishnamoorthy T., Phan L., Pavitt G.D., and Hinnebusch A.G. Conserved bipartite motifs in yeast eIF5 and eIF2Bepsilon, GTPase-activating and GDP-GTP exchange factors in translation initiation, mediate binding to their common substrate eIF2. EMBO J. 18 (1999) 1673-1688
30. Cho S., and Hoffman D.W. Structure of the beta subunit of translation initiation factor 2 from the archaeon Methanococcus jannaschii: a representative of the eIF2beta/eIF5 family of proteins. Biochemistry 41 (2002) 5730-5742
31. Gutierrez P., Osborne M.J., Siddiqui N., Trempe J.F., Arrowsmith C., and Gehring K. Structure of the archaeal translation initiation factor aIF2 beta from Methanobacterium thermoautotrophicum: implications for translation initiation. Protein Sci. 13 (2004) 659-667
32. Sokabe M., Yao M., Sakai N., Toya S., and Tanaka I. Structure of archaeal translational initiation factor 2 betagamma-GDP reveals significant conformational change of the beta-subunit and switch 1 region. Proc. Natl. Acad. Sci. USA 103 (2006) 13016-13021
33. Yatime L., Mechulam Y., Blanquet S., and Schmitt E. Structure of an archaeal heterotrimeric initiation factor 2 reveals a nucleotide state between the GTP and the GDP states. Proc. Natl. Acad. Sci. USA 104 (2007) 18445-18450
34. Hashimoto N.N., Carnevalli L.S., and Castilho B.A. Translation initiation at non-AUG codons mediated by weakened association of eukaryotic initiation factor (eIF) 2 subunits. Biochem. J. 367 (2002) 359-368
35. Thompson G.M., Pacheco E., Melo E.O., and Castilho B.A. Conserved sequences in the beta subunit of archaeal and eukaryal translation initiation factor 2 (eIF2), absent from eIF5, mediate interaction with eIF2gamma. Biochem. J. 347 (2000) 703-709
36. Pedulla, N., Palermo, R., Hasenohrl, D., Blasi, U., Cammarano, P. and Londei, P. (2005) The archaeal eIF2 homologue: functional properties of an ancient translation initiation factor. Nucleic Acid Res. 33, 1804-1812. Print 2005.
37. Schmitt E., Blanquet S., and Mechulam Y. The large subunit of initiation factor aIF2 is a close structural homologue of elongation factors. EMBO J. 21 (2002) 1821-1832
38. Stolboushkina E., Nikonov S., Nikulin A., Blasi U., Manstein D.J., Fedorov R., Garber M., and Nikonov O. Crystal structure of the intact archaeal translation initiation factor 2 demonstrates very high conformational flexibility in the alpha- and beta-subunits. J. Mol. Biol. 382 (2008) 680-691
39. Rajesh K., Iyer A., Suragani R.N., and Ramaiah K.V. Intersubunit and interprotein interactions of alpha- and beta-subunits of human eIF2: effect of phosphorylation. Biochem. Biophys. Res. Commun. 374 (2008) 336-340
40. Suragani R.N., Kamindla R., Ehtesham N.Z., and Ramaiah K.V. Interaction of recombinant human eIF2 subunits with eIF2B and eIF2alpha kinases. Biochem. Biophys. Res. Commun. 338 (2005) 1766-1772
41. Erickson F.L., Nika J., Rippel S., and Hannig E.M. Minimum requirements for the function of eukaryotic translation initiation factor 2. Genetics 158 (2001) 123-132
42. Erickson F.L., and Hannig E.M. Ligand interactions with eukaryotic translation initiation factor 2: role of the gamma-subunit. EMBO J. 15 (1996) 6311-6320
43. Roll-Mecak A., Alone P., Cao C., Dever T.E., and Burley S.K. X-ray structure of translation initiation factor eIF2gamma: implications for tRNA and eIF2alpha binding. J. Biol. Chem. 279 (2004) 10634-10642
44. Nikonov O., et al. New insights into the interactions of the translation initiation factor 2 from archaea with guanine nucleotides and initiator tRNA. J. Mol. Biol. 373 (2007) 328-336
45. Vetter I.R., and Wittinghofer A. The guanine nucleotide-binding switch in three dimensions. Science 294 (2001) 1299-1304
46. Andersen G.R., Valente L., Pedersen L., Kinzy T.G., and Nyborg J. Crystal structures of nucleotide exchange intermediates in the eEF1A-eEF1Balpha complex. Nat. Struct. Biol. 8 (2001) 531-534
47. Berchtold H., Reshtnikova L., Reiser C.O.A., Schirmer N.K., Sprinzl M., and Hilgenfeld R. Crystal structure of active elongation factor Tu reveals major domain rearrangements. Nature 365 (1993) 126-132
48. LaCour T.F., Nyborg J., Thirup S., and Clark B.F. Structural details of the binding of guanosine diphosphate to elongation factor Tu from E. coli as studied by X-ray crystallography. EMBO J. 4 (1985) 2385-2388
49. Phe, EF-Tu, and a GTP analog. Science 270 (1995) 1464-1472
50. Kapp L.D., and Lorsch J.R. GTP-dependent recognition of the methionine moiety on initiator tRNA by translation factor eIF2. J. Mol. Biol. 335 (2004) 923-936
51. Mechulam Y., Guillon L., Yatime L., Blanquet S., and Schmitt E. Protection-based assays to measure aminoacyl-tRNA binding to translation initiation factors. Meth. Enzymol. 430 (2007) 265-281
52. Yatime L., Schmitt E., Blanquet S., and Mechulam Y. Functional molecular mapping of archaeal translation initiation factor 2. J. Biol. Chem. 279 (2004) 15984-15993
53. Manchester K.L. EIF-2B and the exchange of guanine nucleotides bound to eIF-2. Int. J. Biochem. 19 (1987) 245-251
54. Flynn A., Oldfield S., and Proud C.G. The role of the beta-subunit of initiation factor eIF-2 in initiation complex formation. Biochim. Biophys. Acta 1174 (1993) 117-121
55. Mitsui K., Datta A., and Ochoa S. Removal of beta subunit of the eukaryotic polypeptide chain initiation factor 2 by limited proteolysis. Proc. Natl. Acad. Sci. USA 78 (1981) 4128-4132
56. Das A., Bagchi M.K., Ghosh-Dastidar P., and Gupta N.K. Protein synthesis in rabbit reticulocytes. A study of peptide chain initiation using native and beta-subunit-depleted eukaryotic initiation factor 2. J. Biol. Chem. 257 (1982) 1282-1288
57. Stringer E.A., Chaudhuri A., Valenzuela D., and Maitra U. Rabbit reticulocyte initiation factor 2 contains two polypeptide chains of molecular weights 48 000 and 38 000. Proc. Natl. Acad. Sci. USA 77 (1980) 3356-3359
58. Anthony Jr. D.D., Kinzy T.G., and Merrick W.C. Affinity labeling of eukaryotic initiation factor 2 and elongation factor 1 alpha beta gamma with GTP analogs. Arch. Biochem. Biophys. 281 (1990) 157-162
59. Mouat M.F., and Manchester K. An alpha subunit-deficient form of eukaryotic protein synthesis initiation factor eIF-2 from rabbit reticulocyte lysate and its activity in ternary complex formation. Mol. Cell. Biochem. 183 (1998) 69-78
60. Nika J., Rippel S., and Hannig E.M. Biochemical analysis of the eIF2βγ complex reveals a structural function for eIF2α in catalyzed nucleotide exchange. J. Biol. Chem. 276 (2001) 1051-1060
61. von Pawel-Rammingen U., Astrom S., and Bystrom A.S. Mutational analysis of conserved positions potentially important for initiator tRNA function in Saccharomyces cerevisiae. Mol. Cell. Biol. 12 (1992) 1432-1442
62. Drabkin H.J., Helk B., and RajBhandary U.L. The role of nucleotides conserved in eukaryotic initiator methionine tRNAs in initiation of protein synthesis. J. Biol. Chem. 268 (1993) 25221-25228
63. Farruggio D., Chaudhuri J., Maitra U., and RajBhandary U.L. The A1 × U72 base pair conserved in eukaryotic initiator tRNAs is important specifically for binding to the eukaryotic translation initiation factor eIF2. Mol. Cell. Biol. 16 (1996) 4248-4256
64. Wagner T., Gross M., and Sigler P.B. Isoleucyl initiator tRNA does not initiate eucaryotic protein synthesis. J. Biol. Chem. 259 (1984) 4706-4709
65. Marck C., and Grosjean H. TRNomics: analysis of tRNA genes from 50 genomes of eukarya, archaea, and bacteria reveals anticodon-sparing strategies and domain-specific features. RNA 8 (2002) 1189-1232
66. Kapp L.D., Kolitz S.E., and Lorsch J.R. Yeast initiator tRNA identity elements cooperate to influence multiple steps of translation initiation. RNA 12 (2006) 751-764
67. Polekhina G., Thirup S., Kjeldgaard M., Nissen P., Lippmann C., and Nyborg J. Helix unwinding in the effector region of elongation factor EF-Tu-GDP. Structure 4 (1996) 1141-1151
68. Nakakido M., Tanaka Y., Sokabe M., and Tsumoto K. Thermodynamic analysis reveals that GTP binding affects the interaction between the alpha- and gamma-subunits of translation initiation factor 2. Biochem. Biophys. Res. Commun. 371 (2008) 596-599
69. Donahue T.F., and Cigan A.M. Genetic selection for mutations that reduce or abolish ribosomal recognition of the HIS4 translational initiator region. Mol. Cell. Biol. 8 (1988) 2955-2963
70. Castilho-Valavicius B., Thompson G.M., and Donahue T.F. Mutation analysis of the Cys-X2-Cys-X19-Cys-X2-Cys motif in the beta subunit of eukaryotic translation initiation factor 2. Gene Exp. 2 (1992) 297-309
71. Algire M.A., et al. Development and characterization of a reconstituted yeast translation initiation system. RNA 8 (2002) 382-397
72. Laurino J.P., Thompson G.M., Pacheco E., and Castilho B.A. The beta subunit of eukaryotic translation initiation factor 2 binds mRNA through the lysine repeats and a region comprising the C2-C2 motif. Mol. Cell. Biol. 19 (1999) 173-181
73. Siekierka J., Manne V., and Ochoa S. Mechanism of translational control by partial phosphorylation of the alpha subunit of eukaryotic initiation factor 2. Proc. Natl. Acad. Sci. USA 81 (1984) 352-356
74. Rowlands A.G., Panniers R., and Henshaw E.C. The catalytic mechanism of guanine nucleotide exchange factor action and competitive inhibition by phosphorylated eukaryotic initiation factor 2. J. Biol. Chem. 263 (1988) 5526-5533
75. Dever T.E., Yang W., Astrom S., Bystrom A.S., and Hinnebusch A.G. Modulation of tRNA(iMet), eIF-2, and eIF-2B expression shows that GCN4 translation is inversely coupled to the level of eIF-2.GTP.Met-tRNA(iMet) ternary complexes. Mol. Cell. Biol. 15 (1995) 6351-6363
76. Jousse C., Oyadomari S., Novoa I., Lu P., Zhang Y., Harding H.P., and Ron D. Inhibition of a constitutive translation initiation factor 2alpha phosphatase, CReP, promotes survival of stressed cells. J. Cell. Biol. 163 (2003) 767-775
77. Dever T.E., Dar A.C., and Sicheri F. The eIF2α kinases. In: Mathews M.B., Sonenberg N., and Hershey J.W.B. (Eds). Translational control in Biology and Medicine (2007), Cold Spring Harbor Laboratory Press, Cold Spring Harbor 319-344
78. Dar A.C., Dever T.E., and Sicheri F. Higher-order substrate recognition of eIF2alpha by the RNA-dependent protein kinase PKR. Cell 122 (2005) 887-900
79. Dong J., Qiu H., Garcia-Barrio M., Anderson J., and Hinnebusch A.G. Uncharged tRNA activates GCN2 by displacing the protein kinase moiety from a bipartite tRNA-binding domain. Mol. Cell 6 (2000) 269-279
80. Samuel C.E. Mechanism of interferon action. Kinetics of interferon action in mouse L929 cells: phosphorylation of protein synthesis initiation factor elF-2 and ribosome-associated protein P1. Virology 93 (1979) 281-285
81. Meurs E., Chong K., Galabru J., Thomas N.S., Kerr I.M., Williams B.R., and Hovanessian A.G. Molecular cloning and characterization of the human double-stranded RNA-activated protein kinase induced by interferon. Cell 62 (1990) 379-390
82. Harding H.P., Zhang Y., and Ron D. Protein translation and folding are coupled by an endoplasmic-reticulum-resident kinase. Nature 397 (1999) 271-274
83. Shi Y., Vattem K.M., Sood R., An J., Liang J., Stramm L., and Wek R.C. Identification and characterization of pancreatic eukaryotic initiation factor 2 alpha-subunit kinase, PEK, involved in translational control. Mol. Cell. Biol. 18 (1998) 7499-7509
84. Lu L., Han A.P., and Chen J.J. Translation initiation control by heme-regulated eukaryotic initiation factor 2alpha kinase in erythroid cells under cytoplasmic stresses. Mol. Cell. Biol. 21 (2001) 7971-7980
85. Hinnebusch A.G. Translational regulation of GCN4 and the general amino acid control of yeast. Annu. Rev. Microbiol. 59 (2005) 407-450
86. Hinnebusch A.G. Mechanisms of gene regulation in the general control of amino acid biosynthesis in Saccharomyces cerevisiae. Microbiol. Rev. 52 (1988) 248-273
87. Moxley J.F., et al. Linking high-resolution metabolic flux phenotypes and transcriptional regulation in yeast modulated by the global regulator Gcn4p. Proc. Natl. Acad. Sci. USA 106 (2009) 6477-6482
88. Szamecz B., et al. EIF3a cooperates with sequences 5′ of uORF1 to promote resumption of scanning by post-termination ribosomes for reinitiation on GCN4 mRNA. Genes Dev. 22 (2008) 2414-2425
89. Vattem K.M., and Wek R.C. Reinitiation involving upstream ORFs regulates ATF4 mRNA translation in mammalian cells. Proc. Natl. Acad. Sci. USA 101 (2004) 11269-11274
90. Harding H.P., Novoa I., Zhang Y., Zeng H., Wek R., Schapira M., and Ron D. Regulated translation initiation controls stress-induced gene expression in mammalian cells. Mol. Cell 6 (2000) 1099-1108
91. Harding H.P., et al. An integrated stress response regulates amino acid metabolism and resistance to oxidative stress. Mol. Cell 11 (2003) 619-633
92. Costa-Mattioli M., et al. Translational control of hippocampal synaptic plasticity and memory by the eIF2alpha kinase GCN2. Nature 436 (2005) 1166-1173
93. Costa-Mattioli M., Sossin W.S., Klann E., and Sonenberg N. Translational control of long-lasting synaptic plasticity and memory. Neuron 61 (2009) 10-26
94. Hao S., et al. Uncharged tRNA and sensing of amino acid deficiency in mammalian piriform cortex. Science 307 (2005) 1776-1778
95. Maurin A.C., et al. The GCN2 kinase biases feeding behavior to maintain amino acid homeostasis in omnivores. Cell Metab. 1 (2005) 273-277
96. Sonenberg N., and Hinnebusch A.G. Regulation of translation initiation in eukaryotes: mechanisms and biological targets. Cell 136 (2009) 731-745
97. Ron, D. and Harding, H.P. (2007) eIF2α phosphorylation in cellular stress responses and disease. In: Mathews, M.B., Sonenberg, N., Hershey, J.W.B. (Eds.), Translational control in Biology and Medicine, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, pp. 345-368.
98. Alone P.V., and Dever T.E. Direct binding of translation initiation factor eIF2gamma-G domain to its GTPase-activating and GDP-GTP exchange factors eIF5 and eIF2B epsilon. J. Biol. Chem. 281 (2006) 12636-12644
99. Mohammad-Qureshi S.S., Haddad R., Hemingway E.J., Richardson J.P., and Pavitt G.D. Critical contacts between the eukaryotic initiation factor 2B (eIF2B) catalytic domain and both eIF2beta and -2gamma mediate guanine nucleotide exchange. Mol. Cell. Biol. 27 (2007) 5225-5234
100. Conte M.R., Kelly G., Babon J., Sanfelice D., Youell J., Smerdon S.J., and Proud C.G. Structure of the eukaryotic initiation factor (eIF) 5 reveals a fold common to several translation factors. Biochemistry 45 (2006) 4550-4558
101. Fletcher C.M., Pestova T.V., Hellen C.U., and Wagner G. Structure and interactions of the translation initiation factor eIF1. EMBO J. 18 (1999) 2631-2637
102. Pisarev A.V., Kolupaeva V.G., Pisareva V.P., Merrick W.C., Hellen C.U., and Pestova T.V. Specific functional interactions of nucleotides at key -3 and +4 positions flanking the initiation codon with components of the mammalian 48S translation initiation complex. Genes Dev. 20 (2006) 624-636