A method is described to synthesize small RNAs of defined length and sequence using T7 RNA polymerase and templates of synthetic DNA which contain the T7 promoter. Partially single stranded templates which are base paired only in the -17 to +1 promoter region are just as active in transcription as linear plasmid DNA. Runoff transcripts initiate at a unique, predictable position, but may have one nucleotide more or less on the 3' terminus. In addition to the full length products, the reactions also yield a large amount of smaller oligoribonucleotides in the range from 2 to 6 nucleotides which appear to be the result of abortive initiation events. Variants in the +1 to +6 region of the promoter are transcribed with reduced efficiency but increase the variety of RNAs which can be made. Transcription reaction conditions have been optimized to allow the synthesis of milligram amounts of virtually any RNA from 12 to 35 nucleotides in length.

Eukaryotic initiation factor (elF) 4A functions as a subunit of the initiation factor complex elF4F, which mediates the binding of mRNA to the ribosome. elF4A possesses ATPase and RNA helicase activities and is the prototype for a large family of putative RNA helicases (the DEAD box family). It is thought that the function of elF4A during translation initiation is to unwind the mRNA secondary structure in the 5' UTR to facilitate ribosome binding. However, the evidence to support this hypothesis is rather indirect, and it was reported that elF4A is also required for the translation of mRNAs possessing minimal 5' UTR secondary structure. Were this hypothesis correct, the requirement for elF4A should correlate with the degree of mRNA secondary structure. To test this hypothesis, the effect of a dominant-negative mutant of mammalian elF4A on translation of mRNAs with various degrees of secondary structure was studied in vitro. Here, we show that mRNAs containing stable secondary structure in the 5' untranslated region are more susceptible to inhibition by the elF4A mutant. The mutant protein also strongly inhibits translation from several picornavirus internal ribosome entry sites (IRES), although to different extents. UV crosslinking of elF4F subunits and elF4B to the mRNA cap structure is dramatically reduced by the elF4A mutant and RNA secondary structure. Finally, the elF4A mutant forms a more stable complex with elF4G, as compared to the wild-type elF4A, thus explaining the mechanism by which substoichiometric amounts of mutant elF4A inhibit translation.

A cytoplasmic 57-kDa protein that is required for translation of picornavirus RNA by internal ribosomal entry is identical to the nuclear pyrimidine tract-binding protein ( ABSTRACTInitiation of translation of the RNA genomes of picornaviruses such as poliovirus and encephalomyocarditis vius is cap-independent and results from interaction of ribosomes with a segment of the 5' noncoding region of these mRNAs termed the internal ribosomal entry site. Genetic and biochemical studies have previously shown that a 57-kDa cytoplasmic RNA-binding protein (p57) plays an essential rol in this translation mechanism. We have now found that p57 shares physical, biochemical, and antigenic properties with the pyrimidine tract-binding protein (PTB), a nuclear protein that

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