Abstract
Functions of long and short particles of five different tobacco rattle virus (TRV) systems were studied by complementation experiments with the corresponding long and short species of ribonucleic acid (RNA). The progeny of long RNA species alone was proteinless or "free" infectious long RNA, whereas short RNA species alone did not replicate by themselves but appeared to be dependent on long RNA for replication. When both types of RNA derived from the same isolate were inoculated together, particulate virus with long and short particles was produced in more than 50% of the resulting primary infections. These virus systems obtained by homologous complementation resembled the parent isolates in all their characteristics. In addition, heterologous complementation tests were performed with long and short RNA, each derived from another isolate. Heterologous interaction could be observed in only 2 out of 20 possible combinations. As a result, two "mixed" TRV systems with respect to their particle length distributions were obtained, since their long and short particles resembled the ones from the other isolate. The symptoms produced by these mixed viruses were determined by the corresponding long RNA and appeared not to be influenced by the heterologous short one. However, the protein coat of both particles of the "mixed" viruses was specified by the corresponding noninfectious short RNA. Therefore, TRV is a system of at least two functionally defective and mutually complementing components which appear to be specialized in early and late functions.