Karimi R, Pavlov M Y, Heurgué-Hamard V, Buckingham R H, Ehrenberg M
Department of Molecular Biology, BMC, Uppsala, S-75124, Sweden.
J Mol Biol. 1998 Aug 14;281(2):241-52. doi: 10.1006/jmbi.1998.1953.
A novel function of initiation factors IF1 and IF2 in Escherichia coli translation has been identified. It is shown that these factors efficiently catalyse dissociation of peptidyl-tRNAs with polypeptides of different length from the P-site of E. coli ribosomes, and that the simultaneous presence of both factors is required for induction of drop-off. The factor-induced drop-off occurs with both sense and stop codons in the A-site and competes with peptide elongation or termination. The efficiency with which IF1 and IF2 catalyse drop-off decreases with increasing length of the nascent polypeptide, but is quite significant for hepta-peptidyl-tRNAs, the longest polypeptide chains studied. In the absence of IF1 and IF2 the rate of drop-off varies considerably for different peptidyl-tRNAs, and depends both on the length and sequence of the nascent peptide. Efficient factor-catalysed drop-off requires GTP but not GTP hydrolysis, as shown in experiments without guanine nucleotides, with GDP or with the non-cleavable analogue GMP-PNP.Simultaneous overexpression of IF1 and IF2 in vivo inhibits cell growth specifically in some peptidyl-tRNA hydrolase deficient mutants, suggesting that initiation factor-catalysed drop-off of peptidyl-tRNA can occur on a significant scale in the bacterial cell. Consequences for the bacterial physiology of this previously unknown function of IF1 and IF2 are discussed.
已确定大肠杆菌翻译起始因子IF1和IF2的一种新功能。结果表明，这些因子能有效地催化不同长度的多肽从大肠杆菌核糖体的P位点解离肽基-tRNA，并且诱导解离需要这两种因子同时存在。因子诱导的解离在A位点的有义密码子和终止密码子处均会发生，并与肽链延伸或终止相互竞争。IF1和IF2催化解离的效率随新生多肽长度的增加而降低，但对于七肽基-tRNA（研究的最长多肽链）而言，该效率仍相当显著。在没有IF1和IF2的情况下，不同肽基-tRNA的解离速率差异很大，并且取决于新生肽的长度和序列。如在不含鸟嘌呤核苷酸、含GDP或不可裂解类似物GMP-PNP的实验中所示，因子催化的有效解离需要GTP，但不需要GTP水解。在体内同时过表达IF1和IF2会特异性抑制某些肽基-tRNA水解酶缺陷型突变体的细胞生长，这表明起始因子催化的肽基-tRNA解离在细菌细胞中可能会大规模发生。本文讨论了IF1和IF2这种以前未知功能对细菌生理学的影响。
