High-level synthesis of cowpea mosaic virus RNA polymerase and protease in Escherichia coli

Cowpea mosaic virus 32- and 60-kilodalton replication proteins target and change the morphology of endoplasmic reticulum membranes

Cowpea mosaic virus (CPMV) replicates in close association with small membranous vesicles that are formed by rearrangements of intracellular membranes. To determine which of the viral proteins are responsible for the rearrangements of membranes and the attachment of the replication complex, we have expressed individual CPMV proteins encoded by RNA1 in cowpea protoplasts by transient expression and in Nicotiana benthamiana plants by using the tobacco rattle virus (TRV) expression vector. The 32-kDa protein (32K) and 60K, when expressed individually, accumulate in only low amounts but are found associated with membranes mainly derived from the endoplasmic reticulum (ER). 24K and 110K are freely soluble and accumulate to high levels. With the TRV vector, expression of 32K and 60K results in rearrangement of ER membranes. Besides, expression of 32K and 60K results in necrosis of the inoculated N. benthamiana leaves, suggesting that 32K and 60K are cytotoxic proteins. On the other hand, during CPMV infection 32K and 60K accumulate to high levels without causing necrosis.

Replication and translation of cowpea mosaic virus RNAs are tightly linked

The genome of cowpea mosaic virus (CPMV) is divided among two positive strand RNA molecules. B-RNA is able to replicate independently from M-RNA in cowpea protoplasts. Replication of mutant B-transcripts could not be supported by co-inoculated wild-type B-RNA, indicating that B-RNA cannot be efficiently replicated in trans. Hence replication of a B-RNA molecule is tightly linked to its translation and/or at least one of the replicative proteins functions in cis only. Remarkably also for efficient replication of M-RNA one of its translation products was found to be required in cis. This 58K protein possibly helps in directing the B-RNA-encoded replication complex to the M-RNA. In order to identify the viral polymerase the CPMV B-RNA-specific proteins have been produced individually in cowpea protoplasts using CaMV 35S promoter based expression vectors. Only protoplasts transfected with a vector containing the 200K coding sequence were able to support replication of co-transfected M-RNA. Despite this, CPMV-specific RNA polymerase activity could not be detected in extracts of these protoplasts using a poly(A)/oligo(U) assay. These results indicate that, in contrast to the poliovirus polymerase, the CPMV polymerase is not able to accept oligo(U) as a primer and in addition support the concept that translation and replication are linked.

Processing of VPg-containing polyproteins encoded by the B-RNA from cowpea mosaic virus

To study the processing of putative VPg precursors the expression of specific mutant transcripts derived from a full-length cDNA clone of cowpea mosaic virus (CPMV) B-RNA was examined in a rabbit reticulocyte lysate system. This study revealed that the 170K protein produced by a B-RNA mutant that lacks the 32K coding region was efficiently processed by mainly intramolecular cleavages at three different sites into three sets of proteins of 60K + 110K, 84K + 87K, and 58K + 112K. Further cleavage of the 60K protein into 58K and VPg has not been observed in this in vitro system. The 84K protein can be further processed by an intramolecular cleavage reaction via two alternative pathways, either into 26K (VPg + 24K) and 58K proteins or into 24K and 60K proteins. VPg can be released from the 112K (VPg + 110K) precursor either directly or via the 26K intermediate. Immunoblot analysis showed that the 112K protein is present in CPMV-infected plant cells indicating that the in vitro observations may hold true in vivo.

Sequence upstream of the 24K protease enhances cleavage of the cowpea mosaic virus B RNA-encoded polyprotein at the junction between the 24K and 87K proteins

To investigate cleavage at the junction between the cowpea mosaic virus (CPMV) 24K and 87K proteins, plasmids were constructed containing the sequence of bottom-component (B) RNA encoding the 110K protein plus a variable length of upstream coding sequence. Transcripts derived from these clones were translated in rabbit reticulocyte lysate and the appearance of the 87K protein was used to assess the efficiency of cleavage at the 24K-87K junction. The results show that the 110K protein, containing the contiguous sequence of the 24K and 87K proteins, is stable and that efficient cleavage at 24K-87K junction requires the presence of amino acids upstream of the 24K protease. These observations show that the 170K protein rather than the 110K protein is the precursor of the 87K protein and suggest a mechanism whereby both the B RNA-encoded 110K and 87K proteins can accumulate during infection.

Nucleic acid binding properties of the 92-kDa replicase subunit of alfalfa mosaic virus produced in yeast

The 92-kDa non-structural protein of alfalfa mosaic virus (one of the replicase subunits) was synthesized by Saccharomyces cerevisiae transformed with a recombinant expression vector. The yeast-expressed protein had the immunological and size characteristics of the naturally made viral protein. It was partially purified and its nucleic acid binding properties were tested by gel-retardation electrophoresis and nitrocellulose adsorption. The protein interacted with single-stranded RNA, double-stranded RNA and double-stranded DNA in a salt-dependent manner, with a slight preference for RNA. These properties may be related to its putative function as a core RNA polymerase.

Analysis of sequences involved in cowpea mosaic virus RNA replication using site-specific mutants

Using a full-length cDNA clone of cowpea mosaic virus (CPMV) B-RNA from which infectious transcripts can be generated, we examined the influence of a sequence of 11 nucleotides, UUUUAUUAAAA, comprising the nucleotides 5883 to 5893 in the 3' noncoding region of B-RNA, on viral RNA replication. This sequence is not only present in B-RNA but also in M-RNA and represents the 7 nucleotides preceding the poly(A) tail and the first four A residues of the poly(A) tail. Replication of B-RNA transcripts derived from a series of mutants in this region was tested in cowpea plants and protoplasts. Only mutant transcripts with minor modifications appeared able to replicate, which indicates that the region has a function in viral RNA replication. In addition, the results suggest the existence of a hairpin loop in this region. Those transcripts with deletions which disturb the putative hairpin structure have decreased specific infectivities. Mutant transcripts reversed stepwise to the wild-type sequence during replication in plants. This observation strengthens the idea that the sequence of 11 nucleotides has a function in viral RNA replication.