Historically, RNA viruses could be classified on the ability of the naked nucleic acid (RNA separated from the other viral components) to induce a lytic infection. The procedure of transforming cells with viral nucleic acid is known as transfection. Eventually, it was shown that the ability to productively transfect a cell corresponded to the "sense" or polarity of the RNA. Viruses (such as polio) whose genome consisted of message sense (+ or positive sense) single stranded RNA were the only ones found to be able to productively transfect permissive cells. The other viruses (such as influenza) were shown to have genomes that contained the complement to mRNA. In most cases, the missing factor was the absence of the viral polymerase which allowed the complementary RNA to be transcribed into message.

Several complications with this classification system have arisen. 1) In the first place, retroviruses were shown to contain a genome with single stranded message sense RNA which was not infectious. They required reverse transcriptase in order to carry out their complicated replication cycle. 2) An analogous complication is the sense of the reoviruses, which contain message sense RNA as part of their double stranded genome. Enzymatic separation of the strands using a viral protein may be essential for translation. 3) In the third case, some negative ssRNAs (the ssRNA is not infectious) were shown to display ambisense. Ambisense is a situation in which both the genome and its complement contain some coding information. Remember, translation always occurs in the 5' to 3' direction, so the two strands are being translated in opposite directions. Strictly speaking, each strand has regions of + and - polarity, hence ambisense.

Ambisense is seen in the arena family and in one genus of the bunya family, namely the phleboviruses. In many ways, it is still convenient to classify these viruses with the negative ssRNA viruses since they resemble them in terms of virion structure and infectivity.

Copied with permission from Robert Siegel