In one experiment, two viruses used were tobacco
(TMV) and Holmes
rib-grass virus (HRV).
The latter was isolated from Plantago lanceolata
and therefore, is also known as Plantago
strain. Proteins of these two viruses differ in having different frequencies and sequences of amino acids. Also these viruses give different symptoms. On leaves of a particular variety of tobacco, while TMV produces mottling of leaves, HRV produces distinct ring patterns. Reciprocal chimeras using RNA of one strain and protein of the other strain could be obtained. It was found that when these chimeras were used for infection, the progeny had proteins which corresponded to the virus from which RNA of the infecting virus particles was derived. This proved that specificity of virus proteins was determined by RNA alone and that proteins carried no information. Results of these experiments are diagrammatically represented in Figure 25.6.
Subsequently, after the demonstration of genetic nature of RNA in TMV, RNA was found in a number of bacteriophages. Therefore, it may be concluded that nucleic acids in general, RNA as well as DNA, carry genetic information. However, DNA perhaps always functions as genetic material. But RNA in majority of cases is non-genetic; only in specific systems where DNA is absent, it functions as genetic material. In recent years, RNA has been shown to function as enzymes (ribozymes)
also. This aspect of RNA led to award of Nobel Prize in chemistry in 1989 (T. Cech
and S. Altman)
and will be discussed in more detail in Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)