Avian Paramyxovirus - Revisiting The Rule of 'six'

Revisiting The Rule of 'six'

The viral genome consists of a RNA with negative polarity. The lengths of the RNA is unusually constant among the kinds and is very similar, that amount to about 13 KB (Genus Metapneumovirus) to 18 KB (Genus Henipavirus), with most paramyxoviruses usually around 15 KB. Regarding the length of individual members of the Subfamily Paramyxovirinae more exactly, then this follows a regularity unusual with viruses, the divisibility by the number of 6: e.g. Mumpsvirus 15,384 NT, Newcastle Disease virus 15,156 NT. The rule of six describes a requirement for particular viruses to have a genome length with a multiple of six. The viruses that have been proved to prescribe to this rule are the members of the Paramyxoviridae, such as: nipah virus, human parainfluenza virus 2, sendai virus and human parainfluenza virus 3. But, based on simply counting the number of nucleotides within the genome, it could extend to many more viruses within this family and outside of it, like ebola virus.

This multiples of the number of 6 are justified in a special mechanism of the RNA with these viruses. In order for this entire process to be efficient at generating full-length genomic and antigenomic molecules (and hence mRNA molecules), the genome must effectively be enclosed within its protein coat, specifically N proteins. Without this, the virus replication machinery finds it difficult to begin replication. Each N molecule associates with exactly 6 nucleotides, which gets us to the reason as to why these viruses require their genomes to be a multiple of six. If they weren't, genome synthesis would be inefficient and infection may fail and hence natural selection may favour efficient RNA synthesis. It may also have something to do with the position of the RNA bases relative to the N subunits. This being said, not all paramyxoviruses are as stringent about the rule of six as most others. Some are happy at being a bit longer/shorter.