‘Swiss army knife’ secret of RNA polymerase I

The 3-dimensional structure of a molecular machine scientists have likened to the Swiss army knife has been visualised in high-resolution for the first time.

RNA polymerase I incorporates modules which prevent it from having to recruit outside help and explains why the protein works faster than counter-part RNA polymerase II say researchers from the European Molecular Biology Laboratory (EMBL) and Centro de Investigaciones Biológicas.

“Rather than recruiting certain components from outside, RNA polymerase I has them already built in, which explains why it is bigger, and less regulated, but at the same time more efficient,” says Christoph Müller from EMBL, who led the study, published in Nature.

“Because everything is already assembled, there’s no time delay,” explains Maria Moreno-Morcillo, who carried out the work.

Each of the three RNA polymerases makes a specific type of RNA molecule. RNA polymerase II makes mRNA, which carries information encoded in DNA to a ribosome where it can build a protein. RNA polymerase I and III make parts of the machinery which reads mRNA – I builds the RNA that will eventually form ribosomes, while III makes tRNA, which carries protein building blocks to the ribosome for assembly.

RNA polymerase II has been widely studied, and scientists have known its structure and how it works for over a decade, but obtaining a detailed structure of its counterparts has been very difficult. Now the structure of RNA polymerase I has been revealed, scientists can explain some of the protein’s particularities.

RNA polymerase I is much larger than RNA polymerase II – its structure revealed extra modules remarkably similar to other, separate proteins required by RNA polymerase II to do its job. In another part of the molecule, RNA polymerase I has combined what in RNA polymerase II are two separate modules into a single, multi-tasking component.

These changes explain why RNA polymerase I produces RNA at a faster rate, and imply that the cell has fewer ways of controlling RNA polymerase I’s activity since it can’t influence it by changing the availability of helper proteins as it does in the case of RNA polymerase II.

However, RNA polymerase I’s Swiss-army knife strategy provides a solution – a built-in regulatory mechanism which can stop itself from attaching to DNA by bending a loop in its structure to block the space the DNA would usually dock onto.

The structure of the box C/D enzyme reveals regulation of RNA methylation http://www.nature.com/nature/journal/v502/n7472/full/nature12581.html