Time course of alfalfa mosaic virus RNA and coat protein synthesis in cowpea protoplasts.

A study was made of the time course of the synthesis of viral plus-strand RNA, minus-strand RNA, and coat protein in alfalfa mosaic virus-infected Cowpea protoplasts. The three genomic RNAs were synthesized at different rates, as were their corresponding minus-strands. We conclude that viral RNA synthesis is regulated both at the level of minus-strand production and the level of plus-strand production. The synthesis of subgenomic RNA 4 was slower than that of its corresponding genomic RNA (RNA 3), indicating that an additional function, expressed later in infection, is required for production of subgenomic coat protein messenger. The data support a model for RNA 4 synthesis involving internal initiation by the RNA polymerase at the intercistronic junction in minus-strand RNA 3. The temporal relationship of the synthesis of RNA 3, RNA 4, and coat protein is discussed.