Replication and expression of the tospoviral genome.

Sequence analysis of the complete, tripartite RNA genomes of two tospoviruses, tomato spotted wilt virus (TSWV) and impatients necrotic spot virus (INSV), demonstrated that they possess five genes that specify six functional proteins. The negative-stranded L RNA encodes the putative polymerase (TSWV 331.5 kDa; INSV 330.3 kDa), the ambisense M RNA encodes a common precursor to the two glycoproteins (G1 and G2) and a nonstructural protein (NSm), and the likewise ambisense S RNA encodes the nucleocapsid (N) protein and a second nonstructural protein (NSs). These viral proteins are expressed from mRNAs that contain 12–20 nontemplated nucleotides at the 5' ends. This indicates that “cap-snatching” is the mechanism used by the viral polymerase to initiate transcription. Sequence analysis revealed that there was no strict base preference at the endonucleolytic site of the cellular leaders. Whereas the function of NSs, the least conserved tospoviral protein, has remained enigmatic, evidence is accumulating that NSm represents the viral movement protein that is involved in a tubule-guided cell-to-cell movement of nonenveloped nucleocapsids. Protoplasts infected with TSWV or transfected with the NSm gene solely, develop long, NSm-containing tubules that extend from the plasma membrane into the culture medium and are similar to tubules found in plasmodesmata of infected plant tissues. Experiments with transgenic plants confirm that NSm is a plasmodesma-associated protein that can modify intercellular communication in plants.