The location of coat protein of alfalfa mosaic virus (AIMV) strain 425 was determined in protoplasts isolated from infected tobacco leaves and in in vitro inoculated tobacco protoplasts, using immunocytochemistry on ultrathin frozen sections labeled with colloidal gold. In infected tobacco leaves 5 days postinoculation (p.i.) coat protein is present in the cytoplasm and nucleus, especially around the nucleolus. In in vitro inoculated tobacco protoplasts coat protein was not present in the nucleus 6 hr p.i. These results indicate that the presence of coat protein in the nucleus is not necessary for viral replication. However, coat protein could be detected in the nucleus 48 hr p.i. Probably coat protein or virus particles accumulate in the nucleus late in infection. Minus-strand RNA, as part of the replication complex, could be detected in a 650 g pellet fraction of infected tobacco leaves but not in the nucleus, suggesting that replication of AIMV occurs outside the nucleus
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