Polycistronic
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Messenger RNA (mRNA) is said to be polycistronic when it contains the genetic information to translate more than one protein. This is not to be confused with monocistronic mRNAs which yield several gene products by proteolytic cleavage of a polyprotein, since the latter has been synthesized in a single translation process[citation needed].
Polycistronic mRNA is often found in prokaryotes, but rarely in eukaryotes[1]. This is because in eukaryotic translation, ribosomes bind to the 7-methylguanosine 'cap' on the 5' end of the mRNA transcript and scan down until the first AUG is reached[verification needed].
An example of polycistronic mRNA in prokaryotes is in the biosynthetic pathway for tryptophan in E.coli: a single mRNA molecule about 7000 nucleotides long specifies five enzymes, each of which has its own start and stop signals on the mRNA. Another classic example is found in the lac operon, where the lacZ, lacY and lacA genes are transcribed into a single polycistronic mRNA.
Examples of polycistronic mRNA in eukaryotes are often found in the chloroplasts of vascular plants[2]. In addition, transcription of mitochondrial mRNA is polycistronic. These notable exceptions to the otherwise monocistronic nature of nuclear genes in eukaryotes may be interpreted as evidence supporting the endosymbiotic theory.
- ^ Kozak, M. (March 1983). "Comparison of initiation of protein synthesis in procaryotes, eucaryotes, and organelles." (PDF). Microbiological Reviews 47 (1): 1-45. PubMed. Retrieved on 2006-08-12.
- ^ Drapier D, Suzuki H, Levy H, Rimbault B, Kindle KL, Stern DB, Wollman FA (June 1998). "The chloroplast atpA gene cluster in Chlamydomonas reinhardtii. Functional analysis of a polycistronic transcription unit." (PDF). Plant physiology 117 (2): 629-641. PubMed. Retrieved on 2006-08-12.
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