Cotranslational disassembly of tobacco mosaic virus in vitro.

Heterogeneous nucleoprotein preparations of tobacco mosaic virus (TMV), stored at pH 7.0, were treated briefly at pH 8 and then incubated in mRNA-dependent rabbit reticulocyte lysate. The treatment at pH 8.0, under conditions which did not cause detectable polar disassembly as judged by exposure to micrococcal nuclease, caused TMV to stimulate in vitro translation up to 100-fold over background. High-molecular-weight products, characteristic of TMV, appeared, albeit at a significantly slower rate than when naked TMV RNA was used as template. Ribonucleoprotein material from cycloheximide-treated incubations was examined in the electron microscope to reveal a subpopulation of rodlets (approx 5%) with clusters of ribosomes at their concave 5' ends. It is proposed that pH 8 treatment removes the last turn of protective coat protein subunits from the rods and "exposes" at least a 49-nucleotide segment of the efficient 5'-leader sequence of TMV RNA for interaction with 40 S ribosomal subunits. Ribosome translocation during protein synthesis presumably dislodges further coat protein subunits progressively while continuing protection of the incoming TMV RNA.