Mechanism of assembly of tobacco mosaic virus in vitro.

We previously proposed that the assembly reaction of TMV in vitro is initiated by 20S protein aggregate specifically interacting to the 5'-end of TMV-RNA, after which the helical rod grows by the addition of protein subunits. Other workers have reported that the source of protein for the growing helix is also the 20S protein aggregate, but not protein subunits. We now summarize the experimental results that confirm our previous hypothesis. 1) TMV-particle, as gauged by infectivity assay and sucrose gradient analysis, could be formed from PRR under conditions where the formation of 20S protein aggregate could not occur. 2) TMV-particle could be formed from PRR by stepwise addition of NBS-modified protein which is lacking the ability to form 20S protein aggregate. 3) The stable disk aggregate of CGMMV-protein are unable to grow the helical rod. 4) The process of rod elongation by protein subunits was observed directly in electron micrography.