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Two distinct virus-specific functions, i.e., virus assembly and spreading of infection from cell to cell (transport function), are temperature-sensitive (ts) in TMV mutant Ni2519. Assembly of Ni2519 cannot be complemented by the temperature-resistant TMV strains used: A14 (a wild type strain from which Ni2519 was derived) and dolichos enation mosaic virus (DEMV, or cowpea strain of TMV), a thermophilic strain. On the other hand, Ni2519 can serve as a donor of the coat protein to complement is strain Ni118, which has a mutation in the coat protein gene. The genomic RNA can be produced by Ni2519 at a nonpermissive temperature; functionally active Ni2519 coat protein (capable of coating Ni118 RNA upon mixed infection) is produced at a nonpermissive temperature as well. The is phenotype of Ni2519 upon virus assembly probably results not from the ts behavior of any virus-coded protein(s) but is due to the ts properties of the genomic RNA molecule itself, so the possibility of the complementation of assembly of Ni2519 is ruled out. Thus, Ni2519 appears to represent a novel class of virus mutants with is virion RNA. The second is function of Ni2519 (transport of infection) can be complemented by a helper virus. The experimental system used for complementation of the transport function allowed Ni2519 to spread from cell to cell at a nonpermissive temperature. Obviously, Ni2519 infection spreads under these conditions in a form different from that in the mature virions, since its assembly cannot be complemented by the helper virus. Some aspects of the transport function are discussed.

The virion RNA of the temperature-sensitive (ts) TMV mutant Ni2519 was used in experiments on in vitro reconstitution. At 24 degrees RNA of Ni2519 can be reassembled into an infective virus. At 33 degrees the reassembly of Ni2519 RNA results in defective (ribonuclease sensitive) virus particle (DVP) formation. The site of the Ni2519 RNA molecule available for RNase attack is located at a distance of about 600-800 nucleotide residues from the 3'-end in DVP assembled at 33 degrees . It has been shown that it is the initiation (rather than elongation) phase of the virus reconstitution that is temperature-sensitive. The results of an electron microscopic analysis suggest that at least two reconstitution initiation sites (RIS) are operative upon Ni2519 RNA assembly at a nonpermissive temperature (33 degrees). On the contrary, a single RIS is observed in Ni2519 RNA at 24 degrees and in TMV vulgare RNA at 33 degrees. It is suggested that the process of reassembly starts from two initiation sites at 33 degrees and results in the defective particle formation. A model of TMV Ni2519 DVP reassembly at restrictive temperature is offered.