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Transgenic P12 tobacco plants, transformed with the replicase genes P1 and P2 of alfalfa mosaic virus (AIMV), can be infected with RNA 3 of the tripartitite AIMV genome or with a DNA copy of RNA 3 fused to the CaMV 35S promoter and nos terminator. The effect of various modifications on the infectivity of the 35S/cDNA 3 construct to P12 plants was studied. When nonviral sequences ranging from 11 to 200 bp were inserted between the 35S promoter and cDNA 3, the infection became dependent on addition of coat protein (CP) to the inoculum. About 80% of the progeny RNAs resulting from these infections were full-length and had lost the nonviral sequence, whereas 20% were truncated by a deletion of the 5' terminal 79 nucleotides (nt). When the sequence corresponding to the 5' terminal 22 nt of RNA 3 was deleted from the 35S/cDNA 3 construct, the clone was as infectious as the wild type (wt), provided that CP was added to the inoculum, but only progeny RNA with a 5' terminal deletion of 79 nt was produced. The 5' truncated RNA 3 molecules induced necrotic ringspot-like symptoms on P12 tobacco plants, whereas wt RNA 3 did not induce detectable symptoms on these plants. It is proposed that in the infections with the modified 35S/cDNA 3 clones, CP is required in the inoculum to permit internal initiation of plus-strand RNA 3 synthesis on 3'-extended or 3'-truncated minus-strand RNA templates. Evidence was obtained that minus-strand RNA 3 synthesized under the control of the 35S promoter was not infectious to P12 plants.

The leader sequence of RNA 3 of the Leiden isolate of alfalfa mosaic virus strain 425 consists of 345 nucleotides (nt) and contains four putative stem-loop structures each with a motif in the loop that resembles the internal control region 2 (ICR2) of tRNA genes. The sequence of the 5' terminal 112 nt of this leader contains one of these stem-loop structures and is sufficient for a reduced accumulation of RNA 3 in protoplasts and a delayed accumulation in plants (E. A. G. van der Vossen et al., Nucleic Acids Res. 21, 1361-1367 (1993). A number of mutations were made in this 112-nt leader sequence to investigate its role in RNA 3 accumulation. Deletion of nucleotides 23-43, 44-90, or 55-112 and inversion or duplication of nucleotides 44-90 all abolished RNA 3 accumulation. Similarly, two base substitutions in the ICR2 motif (nucleotides 60-77) abolished RNA'3 accumulation. Mutations in the stem sections of the putative stem-loop structure had various effects on RNA 3 accumulation and supported the notion that this structure is important for plus-strand promoter activity.

RNA 3 of alfalfa mosaic virus (AIMV) encodes the movement protein P3 and the viral coat protein (CP) which is translated from the subgenomic RNA 4. RNA 3 is able to replicate in tobacco plants transformed with the AIMV replicase genes P1 and P2 (P12 plants). Frameshifts or deletions in the P3 gene have little effect on RNA 3 accumulation in P12 protoplasts whereas such mutations in the CP gene result in a 100-fold reduction of plus-strand RNA 3 accumulation. When P12 protoplasts were inoculated with a mixture of a RNA 3 mutant with a deletion in the P3 gene and a mutant with a deletion in the CP gene, CP expressed by the P3 mutant was unable to upregulate plus-strand RNA accumulation of the CP mutant. However, when a wild-type CP gene and subgenomic promoter were inserted in a RNA 3 mutant with a defective CP gene, the mutant accumulated at wild-type levels. It is concluded that the function of CP in plus-strand RNA 3 accumulation acts in cis and cannot be complemented in trans. In P12 plants, P3 and CP mutants were able to complement each other at low and variable levels. This complementation in plants appeared to be correlated with the occurrence of recombination to wild-type RNA 3.

RNA 3 of alfalfa mosaic virus encodes the movement protein P3 and the viral coat protein (CP). CP is translated from a subgenomic (sg) messenger, RNA 4. To characterize the sg promoter that is responsible for RNA 4 synthesis in vivo, putative sg promoter sequences were inserted in a unique Xhol site located between the initiation codon of the P3 gene and a second in-frame ATG codon in an infectious cDNA clone of RNA 3. Mutants with an active sg promoter insert expressed an N-terminally truncated P3 protein and were able to accumulate in plants. In addition, sg promoter activity was analyzed in protoplasts. When the transcription start site is taken as +1, the sequence of nucleotides -26/+1 was found to have a basal level of sg promoter activity. This activity was increased to near maximum levels when the sg promoter sequence was extended to -136/+12. The upstream positive regulatory element was mapped to nucleotides -136/-94. Engineering of point mutations and small deletions in RNA 3 around the transcription start site for RNA 4 synthesis revealed elements important for sg promoter activity with similarity to sequences conserved in sg promoters of alpha-like viruses. Some of these elements appeared to be required in cis for RNA 3 accumulation. A deletion of the C-terminal three amino acids of the P3 protein rendered this protein nonfunctional in cell-to-cell movement.

To investigate the role of alfalfa mosaic virus coat protein (CP) in genome activation, asymmetric plus-strand RNA accumulation, and cell-to-cell spread of the virus, mutations were made in the CP gene and putative CP binding sites in the 3'-untranslated region (UTR) of RNA 3. Mutants that produced no CP-related peptide or CP with an N-terminal deletion of 20 amino acids were defective in all three functions. Insertion of several nonviral amino acids at position 85 of CP had little effect on genome activation and plus-strand RNA accumulation but abolished cell-to-cell spread. A mutant encoding CP with a C-terminal deletion of 21 amino acids was defective in plus-strand RNA accumulation but showed substantial levels of genome activation and cell-to-cell spread. Mutations in the 3'-UTR that interfered with CP binding affected plus-strand RNA accumulation and cell-to-cell spread. Neither CP nor CP binding sites at the 3'-end of RNA 3 were required for minus-strand RNA accumulation. The results demonstrate that early and late functions of CP can be mutated separately, indicating that different domains of CP are involved in the three functions investigated.