Visualization by atomic force microscopy of tobacco mosaic virus movement protein-RNA complexes formed in vitro

The structure of complexes formed in vitro by tobacco mosaic virus (TMV)-co ded movement protein (MP) with TMV RNA and short (890 nt) synthetic RNA tra nscripts was visualized by atomic force microscopy on a mica surface. MP mo lecules were found to be distributed along the chain of RNA and the structu re of MP-RNA complexes depended on the molar MP:RNA ratios at which the com plexes were formed. A rise in the molar MP:TMV RNA ratio from 20:1 to 60-10 0: 1 resulted in an increase in the density of the MP packaging on TMV RNA and structural conversion of complexes from RNase-sensitive 'beads-on-a-str ing' into a 'thick string' form that was partly resistant to RNase. The 'th ick string'-type RNase-resistant complexes were also produced by short synt hetic RNA transcripts at different MP:RNA ratios. The 'thick string' comple xes are suggested to represent clusters of MP molecules cooperatively bound to discrete regions of TMV RNA and separated by protein-free RNA segments.