Evidence that nucleocapsid disassembly and a later step in virus replication are inhibited in transgenic tobacco protoplasts expressing TMV coat protein

Tobacco mosaic virus (TMV)-like pseudovirus particles containing mRNA for Escherichia coli beta-glucuronidase (GUS) were electroporated into mesophyll protoplasts from control or TMV coat protein (CP)-transgenic tobacco (Nicotiana tabacum cv. Xanthi). GUS-particles were expressed 100-fold less efficiently in CP-transformed than in control protoplasts whereas unencapsidated GUS mRNA was expressed only 2.8-fold less efficiently. Lower transient expression of packaged GUS mRNA is probably due to inhibited disassembly of nucleocapsids in CP-transgenic protoplasts. Control and U1 CP-transformed protoplasts are equally susceptible to infection by cowpea strain TMV (Cc), as well as unencapsidated Cc or U1 RNA. In contrast, native or in vitro reconstituted U1 TMV particles result in 5- to 6-fold fewer infected CP-transgenic than control protoplasts. When Cc RNA was transcapsidated in U1 CP in vitro, the hybrid virions were equally infectious in both classes of protoplasts. We conclude that although compatible U1 protein-protein interactions significantly inhibit (GUS) nucleocapsid disassembly in CP-transgenic protoplasts, the endogenous CP must also interfere with a later stage of infection involving the homologous viral RNA.