Figure 3.

Creating a wide range of variable −1 PRF signals/resistors (rheostats and potentiometers) using trans-acting factors. A, trans-acting nucleic acid–derived elements. A variety of possible −1 PRF elements are shown, graphed relative to ranges of −1 PRF that they can promote. In general, more stable downstream stimulatory elements promote higher levels of −1 PRF and thus serve to increase resistor strength. For example, simple stem-loops (SL) are less thermodynamically stable and thus typically promote low levels of −1 PRF (in the range of 1%); these can be used as weak resistors. In contrast, mRNA pseudoknots (ΨK) are more stable, promote higher rates of −1 PRF (in the range of 10%), and can thus be used as stronger resistors. Trans-acting RNAs, DNAs, or synthetic nucleic acid-derived oligomers can be variously employed to create pseudo-stem-loops (ΨSL) (19, 20), pseudo-pseudoknots (ΨΨK) (21), pseudoknots in combination with micro-RNAs (ΨK⁺ miR) (22), and potentially even pseudo-pseudo-pseudoknots (ΨΨΨK), thus generating a wide range of resistors. B, −1 PRF stimulated by the encephalomyocarditis virus protein 2A as an example of −1 PRF stimulated by a trans-acting protein (25).