To a dump and back
Every week it delivers new surprises to the research community: RNA just doesn't rest - in the literal sense. Because even from the garbage she comes back to life.
The messenger boy has to put up with a lot. As soon as he is born, he is dismembered and patched up again shortly afterwards. He is forced out of the house where he was born through a narrow hole. Then fat henchmen, who always keep the little courier in a stranglehold, drag him on to the place where he is supposed to deliver his message.
But he can't get enough air there either. Without hesitation, a few dozen powerful inquisitors immediately jump on him to squeeze the message out of him. And when they're finally done with it and know everything they need, they grab it again - and off they go with the gnome to the heap, where, exhausted, it ends its short life.
The messenger would certainly have our fullest sympathy - if it weren't just a molecule: namely the messenger or messenger RNA - mRNA for short. The rude treatment of her is partly explained by the fact that she is not just a messenger, but a message herself. And once she has delivered her message - that is, herself - the cell's henchmen make short work of her before she causes any more confusion.
The existence of every mRNA begins deep in the cell nucleus as a copy of a gene in DNA. Their destination is the site of translation – the ribosomes. These translate the nucleic acid sequence extremely quickly, amino acid by amino acid, into a protein. In order to use the information several times, a whole series of ribosomes attach themselves to the mRNA chain. Cell biologists refer to the structure visible under the electron microscope as a polysome.
The fact that the messenger then ends up in a kind of rubbish dump has only recently become known. In 2003, Roy Parker of the Howard Hughes Medical Institute discovered small spots scattered throughout the cytoplasm where used-up mRNAs and RNA-degrading enzymes are deposited. He called the blobs "P-Bodies".
The name came about as a makeshift solution – P-Bodies stands for Processing Bodies, which actually means nothing more than: "Something is happening there." But what exactly is still far from clear.
The research results now presented by Parker suggest that the P-bodies are not exclusively an RNA graveyard. Because sometimes the exhausted messengers seem to be dragged away from there again to be interrogated once more on the ribosomes. At least that's what happens with baker's yeast, as Parker and his colleagues found out. This recycles the old gene transcripts from the mRNA rubbish bin in order to use them to produce new proteins [1].
The fate of a messenger RNA – to the polysome or to the P-body – is determined by the costume in which it appears: on its way from the cell nucleus to the ribosomes, it usually has a guanine cap at the front and a long tail at the back attached by adenine molecules. Then, during translation, the adenine tail shortens-leaving its feathers, so to speak-and the messenger RNA is sent to the P-bodies for disposal. However, when the scientists there blocked the removal of the cap, the molecules soon returned to the riobosomes from the trash can.
Together with his colleague Jeff Coller, Parker also discovered the proteins that operate the decostation of the RNA or counteract it - and thus decide on degradation or recycling [2]. Gregory Hannon of the Cold Spring Harbor Laboratory recently identified the P-bodies as the sites where the much-publicized RNA interference (RNAi) - the process by which tiny snippets of RNA block the reading of specific genes - takes place. Accordingly, the P-bodies would not be a central dump for all mRNAs, but only for those switched off by RNAi - sort of for hazardous waste [3].
In any case, Parker is certain that the P-bodies have a much more far-reaching significance for the cell than previously assumed and speaks of an mRNA cycle because of the possible double function as a garbage dump and store. But even if the RNA messengers return after their rapid march through the cellular institutions - the comeback will probably be short-lived because mRNA is a rather unstable substance.
