Translation internal initiation

Borman, A. M., and Kean, K. M. (1997). Intact eukaryotic initiation factor 4G is required for hepatitis A virus internal initiation of translation. Virology 237, 129-136. 

Iizuka, N., Najita, L., Franzusoff, A., and Sarnow, P. (1994). Cap-dependent and cap-independent translation by internal initiation of mRNAs in cell extracts prepared from Saccharomyces cerevisiae. Mol. Cell Biol. 14, 7322-7330. 

Test hits for their ability to inhibit cap-dependent translation and internal initiation from poliovirus, EMCV, HCV, and CrPV IRES... 

Lomakin, I. B., Hellen, C. U., and Pestova, T. V. (2000). Physical association of eukaryotic initiation factor 4G (eIF4G) with eIF4A strongly enhances binding of eIF4G to the internal ribosomal entry site of encephalomyocarditis virus and is required for internal initiation of translation. Mol. Cell Biol. 20, 6019-6029. 

Hunt, S. L., Hsuan, J. J., Totty, N., and Jackson, R. J. (1999). Unr, a cellular cytoplasmic RNA-binding protein with five cold-shock domains, is required for internal initiation of translation of human rhinovirus RNA. Genes Dev. 13, 437—448. 

Pelletier, J. and Sonnenberg, N. (1988) Internal initiation of translation of eukaryotic mRNA directed by a sequence derived from poliovirus RNA. Nature 334, 320-325. 

From the available data collected from studies of a variety of plant and animal viral RNAs, it appears that initiation is usually limited to a region of the mRNA near the 5 terminus of the chain, althou the maximum allowable distance has not been defined. Thus, if an internal initiation site does exist on poliovirus RNA, it would appear to be an exception to the general preference for sites located near the 5 end. If the proximity of the initiation site to the 5 ond is usually determined by a requirement to be near the cap group, however, this restriction may not apply to an "uncapped mRNA. At this time, there is no information regarding the location of the two initiation sites utilized for translation of poliovirus RNA in vitro. 

Originally two alternative models were proposed for the internal initiator, P2 [134,135]. One model pictured P2 as a special translation initiator, and the other considered P2 as a transcription initiator or promoter. Morse and Yanofsky [137] concluded that in E. coli it was... 

In a special class of internal initiations, the ribosomes bind at a specific internal AUG codon to start translation. The internal AUG may be located as far as 800 bases from the 5 end as in picornaviruses with uncapped, (+)-sense genomic RNAs. According to the model of cap-independent translation or internal ribosome entry (55), the internal RBS or ribosome landing pad is a 5 UTR sequence that forms a stable stem-loop structure. Hepatitis B virus (HBV) and some retroviruses also use an internal AUG codon, rather than ribosomal frameshifting, to express their reverse transcriptase genes (56,57). 

The frequency (number per second) of /transitions from all g. degenerate initial internal states and from the p. d E. initial external translational states is equal to tire reverse frequency from the g degenerate final internal states and the pyd final external translational states. The detailed balance relation between the forward and reverse frequencies is therefore... 

A theoretical upper limit of initial fragment velocity can be calculated if it is assumed that the total internal energy E of the vessel contents is translated into... 

Figure 38-10. Picornavimses disrupt the 4F complex. The 4E-4G complex (4F) directs the 40S ribosomal subunit to the typical capped mRNA (see text). 4G alone is sufficient for targeting the 40S subunit to the internal ribosomal entry site (IRES) of viral mRNAs. To gain selective advantage, certain viruses (eg, poliovirus) have a protease that cleaves the 4E binding site from the amino terminal end of 4G. This truncated 4G can direct the 40S ribosomal subunit to mRNAs that have an IRES but not to those that have a cap. The widths of the arrows indicate the rate of translation initiation from the AUG codon in each example.

The catalytic activities of the fortified wheat germ cell-free systems supplemented with each fraction were investigated (Fig. 2). As shown in Fig. 2, only 0 - 40 % ammonium sulfate fraction showed an enhancement in DHFR protein synthesis. This enhancement of protein experimental results and the fact that the various eukaryotic initiation factors are contained in synthesis was also confirmed by SDS-PAGE and autoradiography (Fig. 3). From the above 0-40 % ammonium sulfate fraction [5, 6], it can be concluded that the amount of initiation factors in a conventionally prepared wheat germ cell-fi extract is deficient for the translation of DHFR with internal ribosome entry site. Therefore, it needs to supplement a wheat germ cell-free extract with the fraction containing the limited initiation factors for the efficient protein translation, and this fortified cell-free system can be easily made by simple... 

Table 6.1 Translation initiation factor requirements of internal ribosome entry site (IRES) elements... 

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