Alfalfa mosaic virus temperature-sensitive mutants IV. Tbts 7, a coat protein mutant defective in an early function

Abstract

Tbts 7 is coat protein mutant of alfalfa mosaic virus (AIMV) which replicates in tobacco leaf disks at 23° but not at 30°, whereas the wild-type (wt) replicates at both temperatures. To analyze the temperature-sensitive step in virus multiplication, the replication of Tbts 7 in protoplasts was investigated. The data indicated that poly- l-ornithine (PLO), when present in the inoculum, selectively interfered with the uncoating in cowpea protoplasts of the mutant virions containing RNA 3 (top component b, Tb) whereas uncoating of virions containing RNAs 1 and 2 was not noticably affected. When PLO in the inoculum was replaced by polyethylene glycol (PEG), Tbts 7 was able to replicate in cowpea and in tobacco protoplasts at 25° but not at 30°. At the restrictive temperature the replication of mutant RNA 3 was selectively defective. An infection induced by an inoculum containing a mixture of Tbts 7 genomic RNAs and wt coat protein was similarly temperature sensitive in RNA synthesis, indicating that a defect in uncoating of Tb is not responsible for the temperature-sensitive behavior. Addition of wild-type Tb to a Tbts 7 inoculum restored the wt phenotype. Under these conditions, at both 25 and 30°, only wt RNA 3 was replicated and not the mutant RNA 3 present in the inoculum. The results are discussed in respect of the early function of AIMV coat protein in the virus replication cycle.