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The cell nucleus is a library of DNA, which is not much use for anything active, like splitting sugars or trapping light. It just sits there, glaring.

So it has to be translated into another form, that of RNA - which is much like DNA (only one letter different...) only snazzier and more zippy about the place. One of RNA's roles is to be further translated into everyone's favourite substance - protein.

However the fun does not stop there; RNA also helps out in other cellular dramas. Confusingly, the processes of converting the sequence of nucleotides to protein (via RNA) itself relies on different types of this versatile molecule. This circularity of cause and effect is commonplace in biology.

All this activity springs from the printing press that is RNA polymerase : DNA goes in, and RNA emerges. Since both are quite bulky ropes of molecules, the enzyme is a large complex of around 10 proteins that associate together into a large 'machine'. Of course, any particular polymerase type has exactly, say, 12 subunits - but there are several types.

Most eukaryotes have a measly three types of polymerase (called pol I, pol II, and pol III). Mighty plants have 4 (as discovered in 2005 in Washington University in St. Louis, USA). The researchers think this 'pol IV' (every even pol is a good one) is dutifully making special RNA that mucks around with the DNA in the nucleus. Specifically, it may be involved in DNA methylation.

Onodera, Yasuyuki, Jeremy Haag, Thomas Ream, Pedro Costa Nunes, Olga Pontes and Craig S. Pikaard (2005). Plant Nuclear RNA polymerase IV mediates siRNA and DNA methylation-dependent heterochromatin formation. Cell 120:613-622