CONTROL OF TRANSLATION BY MESSENGER-RNA SECONDARY STRUCTURE - THE IMPORTANCE OF THE KINETICS OF STRUCTURE FORMATION

RNA secondary structure is important in a wide variety of biological p rocesses, but relatively little is known about the pathways and kineti cs of RNA folding. When the IS 10 transposase (tnp) gene is transcribe d from a promoter outside the element, little increase in tnp expressi on is observed. This protection from outside transcription (pot) occur s at the translational level, presumably resulting from mRNA secondary structure proposed to sequester the tnp ribosome-binding site. Here, we confirm the pot RNA structure and show that it blocks 30S ribosomal subunit binding in vitro. Point mutations that abolish protection in vivo map to the pot structure, Surprisingly, these pot mutations do no t severely alter the pot secondary structure or increase 30S subunit b inding in vitro, except in one case, Using an oligonucleotide hybridiz ation assay, we show that most of the pot mutations slow the kinetics of pot structure formation, with little or no effect on the inhibitory function of the final structure. Moreover, a suppressor mutation reve rses this effect. We propose a pathway for pot mRNA folding that is co nsistent with the mutations and implicates the formation of important kinetic intermediates. The signficance of these observations for the R NA folding problem in general is discussed.