Pon C L, Gualerzi C
Proc Natl Acad Sci U S A. 1974 Dec;71(12):4950-4. doi: 10.1073/pnas.71.12.4950.
Under certain conditions, initiation factor 3 (IF-3) can cause the release of aminoacyl-tRNA bound to 30S ribosomal subunits of E. coli. It is shown that this IF-3-induced aminoacyl-tRNA release cannot be attributed to either nucleolytic attack or competition between IF-3 and aminoacyl-tRNA for the same ribosomal binding site. It was found that the 30S-aminoacyl-tRNA-codon complexes formed in the absence of IF-3 are intrinsically different from those prepared in the presence of IF-3. In the absence of IF-3, the ribosomal binding of aminoacyl-tRNA is a virtually irreversible process, since the bound aminoacyl-tRNA can neither be spontaneously released upon dilution nor exchanged for unbound aminoacyl-tRNA. In the presence of IF-3, the binding of one molecule of IF-3 per 30S ribosome renders the binding of aminoacyl-tRNA reversible upon dilution and promotes exchange between bound and unbound aminoacyl-tRNA. It is suggested that this difference is due to a conformational transition of the 30S ribosomal subunit induced by the binding of IF-3. The possible implications of this finding in relation to the mechanism of action of IF-3 and its functional role in the cell are discussed.
在某些条件下，起始因子3（IF-3）可导致与大肠杆菌30S核糖体亚基结合的氨酰-tRNA释放。结果表明，这种由IF-3诱导的氨酰-tRNA释放既不能归因于核酸酶攻击，也不能归因于IF-3与氨酰-tRNA对同一核糖体结合位点的竞争。研究发现，在没有IF-3的情况下形成的30S-氨酰-tRNA-密码子复合物与在有IF-3的情况下制备的复合物本质上不同。在没有IF-3的情况下，氨酰-tRNA与核糖体的结合实际上是一个不可逆的过程，因为结合的氨酰-tRNA在稀释时既不能自发释放，也不能被未结合的氨酰-tRNA交换。在有IF-3的情况下，每30S核糖体结合一分子IF-3会使氨酰-tRNA在稀释时的结合变得可逆，并促进结合态和未结合态氨酰-tRNA之间的交换。有人认为，这种差异是由于IF-3结合诱导的30S核糖体亚基构象转变所致。本文讨论了这一发现与IF-3作用机制及其在细胞中的功能作用的可能关系。
