Today I heard about a phenomenon called "translational coupling", where the translation of one protein influences the translation of another protein. The messenger RNA levels don't seem influenced. How does this work? Do they need to be in the same operon?
Translational coupling describes in how some cases an mRNA will code from more than one protein (i.e. will be polycistronic). Translational coupling is thought to be mostly used as a way to make a set of genes are translated at roughly the same amount in the cell.
Translational coupling is very common in prokaryotes and nearly half of e coli genes are found in a polycistronic operon. What we know about them is revealing. There are some fancy mechanisms to adjust the ratios of these adjacent genes, which are still coupled, but with ratios that are not just 1:1. Its shown that the later genes are often translated at somewhat lower frequency because the first genes are available more quickly before the mRNA degrades.
I've only been able to find this reference to a eukaryotic case of "translational coupling" which is very rare, but does exist. The most common cases of translationally coupled genes in eukaryotes are RNA viruses which usually contain only a single full length mRNA which codes for all the genes in the virus. The selective pressures to keep the viral genome small and constrain the ratios of these genes, will cause these genes to even overlap, starting the next one before the current gene finishes.
The above answer isn't entirely accurate. It's true that polycistronic mRNAs (i.e. operons) are very common in bacteria. But this isn't what is meant by translationally coupled.
Translational coupling happens when the second gene of two adjacent genes in an operon, doesn't have it's own ribosome binding site. Instead, the ribosome ends translation at a stop codon at the first gene, and then steps back slighly, and starts translating the from the start codon of the second gene. This happens when the coding regions of the two genes overlap:
E.g. XXXXXXXXXATGAXXXXXXXXX -the first series of Xs is the end of gene 1 - the TGA is the stop codon for gene 1 - the ribosome slides back one nucleotide and translates form ATG - the second series of XXXX is the second genes coding region
The two genes are said to be translationally coupled because distrupting translation of the first gene would abolish translation of the second.