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.
Eric Alm @ MIT wrote this great paper on how operons evolve.
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.
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