Reassembly of TMV 20-S protein disks with 3-S RNA fragments

Abstract

The 3-S fragments (3-S RNA) derived from TMV RNA by limited hydrolysis with pancreatic RNase are capable of forming a stable nucleoprotein complex (NPC) with the 20-S double disks of TMV protein. The s20,w of this NPC was of 30–32 S (30-S NPC).Electron microscopic and optical diffraction analyses suggest that 30-S NPC is a short helix with the diameter of intact TMV built up of 4–6 pitches. The 30-S NPC contains about 2.5% RNA.The protein subunits in 30-S NPC acquire antigenic identity with intact TMV. Thus, the interaction of 3-S RNA with 20-S disks induces a conformational shift in the protein subunits including their surface (antigenic) sites. The encapsidation of 3-S RNA into 30-S NPC is accompanied by a shift in the CD spectrum of the former, similar to that reported to occur upon TMV RNA reconstitution.Although it is thought that a TMV RNA molecule possesses at least one (5′-terminal) reconstitution-initiating site (RIS) displaying a high affinity for TMV protein, the experiments on the reassembly of 20-S disks with TMV RNA fragments of different S values suggested that the number of RIS-possessing (or, maybe, RIS-simulating) fragments increased upon the fragmentation of RNA into fragments smaller than 14 S (MV about 0.42 × 106). These observations taken together with the experiments on mixed reassembly (with heterologous intact RNA's and 3-S RNA) suggest that the specificity of 20-S disk interaction with fragmented RNA is not very high, i.e., TMV protein is capable of interacting with 3-S RNA fragments of differing nucleotide sequences.