The structure of complexes formed in vitro by tobacco mosaic virus (TMV)-coded movement protein (MP) with TMV RNA and short (890 nt) synthetic RNA transcripts was visualized by atomic force microscopy on a mica surface. MP molecules were found to be distributed along the chain of RNA and the structure of MP-RNA complexes depended on the molar MP : RNA ratios at which the complexes were formed. A rise in the molar MP : TMV RNA ratio from 20 : 1 to 60-100 : 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-string ' into a ' thick string ' form that was partly resistant to RNase. The ' thick string '-type RNase-resistant complexes were also produced by short synthetic RNA transcripts at different MP : RNA ratios. The ' thick string ' complexes are suggested to represent clusters of MP molecules cooperatively bound to discrete regions of TMV RNA and separated by protein-free RNA segments.

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