<P> In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter . The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans - splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product . The result of this is that the genes contained in the operon are either expressed together or not at all . Several genes must be co-transcribed to define an operon . </P> <P> Originally, operons were thought to exist solely in prokaryotes (which includes organelles like plastids that are derived from bacteria), but since the discovery of the first operons in eukaryotes in the early 1990s, more evidence has arisen to suggest they are more common than previously assumed . In general, expression of prokaryotic operons leads to the generation of polycistronic mRNAs, while eukaryotic operons lead to monocistronic mRNAs . </P> <P> Operons are also found in viruses such as bacteriophages . For example, T7 phages have two operons . The first operon codes for various products, including a special T7 RNA polymerase which can bind to and transcribe the second operon . The second operon includes a lysis gene meant to cause the host cell to burst . </P>

What is the advantage of having just one promoter for all structural genes