For every rule in biology there is an exception. This causes problems in teaching. Do you give the rule and leave out the exception, effectively lying to the people you are teaching or do you give them the rule and exception and making rule mean much less? This is a real problem. Well I have just discovered eukaryotes have broken a golden. They have operons. I was always taught eukaryotes don’t have operons, only prokaryotes do.
For anyone needing to be reminded what an operon is it is several genes all on the same mRNA molecule. One name for this is polycistronic mRNA (cistron is an old name for what we know as a protein coding gene). Several genes have a single promoter and a terminator of transcription controlling them all. On this polycistronic mRNA are several Shine-Dalgarno sequences (bacterial ribosome binding sites) so a ribosome binds in front of each gene and translates it until it reaches a stop codon and falls off. The classic example is the lac operon. It makes 3 proteins used by E. coli to break down the sugar lactose. The reason why you would want a single promoter to control the expression of several proteins is because if they have related function like in the lac operon you want them all to be turned on at the same time and at the same level of expression therefore making the same number of proteins.
Classically eukaryote have monocistronic mRNA (one gene per mRNA) but their is more to this story. Strange transcripts have been found in animals from the simple nematodes to complex mammals such as us. One type of mRNA that has been called operons by some people in C. elegans uses something called trans-splicing. A gene is expressed with one promoter but contains 2 protein coding regions. But this does not mature into a polycistronic mRNA but during RNA processing the introns are removed and the two coding regions are separated. But how does the downstream RNA resist degradation by RNases? It has a SL-exon from another transcript trans-spliced onto it. This way one ‘gene’ makes 2 mRNA (see diagram). This is not a conventional operon like those found in bacteria. This is not a minor thing in the nematodes genome – 15% of all genes in C. elegans are found in operons of this type. We still have monocistronic mRNA here so eukaryotes are behaving fairly well so far.
But in Drosophila and higher organisms dicistronic mRNAs have been found. No trans-splicing involved what so ever. This is definitely breaking one of the golden rules in biology. Here a promoter causes the transcription of 2 genes onto the same transcript. Each one of these gets translated. It is unclear at the moment if both genes have there own ribosome intonation site are if the ribosome that translates the first gene also translates the second one by not falling off at the stop codon of the first gene. One example (that is conserved between man and mouse) is the dicistronic mRNA coding the growth and differentiation factor 1 (GDF-1) and a membrane protein of unknown function (UOG-1). Whether they are co-transcribed because they have related function is obviously unknown.
Living creates are strange. We think we understand a bit about them, create rules that we think life follows so we can understand life better. But life doesn’t always follow the rules we think it does. So the best we can do is make rules or models up and test them as this is how science works. I just hate it when something I thought I new turned out to be wrong.
T. Blumenthal (2004). Operons in eukaryotes Briefings in Functional Genomics and Proteomics, 3 (3), 199-211 DOI: 10.1093/bfgp/3.3.199