Assembly of Tobacco Mosaic Virus in vitro: Effect of State of Polymerization of the Protein Component

We previously proposed that the assembly of tobacco mosaic virus is initiated by disks of 34 protein subunits attaching to the RNA, after which the particle grows by the addition of further disks. Other workers have reported growth from ``A-protein'' instead. We now report experiments that confirm our previous results and show that the contrary findings are largely due to a nonequilibrium form of A-protein that has a ``memory'' for the disk state and rapidly reaggregates, either in solution or on the growing particle.