Monday, November 16, 2009

Ciliate transposases redux

I got the latest issue of Genes & Development in the mail last week, which would be unremarkable except that I don't actually subscribe to Genes & Development. Weird but useful, since this issue of G&D has an article on a "domesticated" transposase in Paramecium tetraurelia that's used to excise internal eliminated sequences (IESs) when converting the micronuclear genome into a macronuclear genome. What in the world am I talking about? Some might recall my comments on the transposases of Oxytricha, where I explained that the ciliate genome is present in two strikingly different forms. One form is a visibly large nucleus called the macronucleus, where all the transcription and gene expression is going on. The other genome is found in a compact little storage nucleus called the micronucleus, which is used in sexual reproduction. After sexual reproduction, a new macronucleus is generated from the micronucleus by excising a huge amount of DNA to generate actual genetic reading sequences, which are disrupted by IESs in the micronuclear genome. It's an amazing and frankly baffling genomic cycle.

The earlier study I commented on implicated a transposase as an important factor used to excise the IESs during development of the new macronucleus in the ciliate Oxytricha. This latest study, by Baudry et al. looked at the activity of a gene they call PiggyMac, which is homologous to piggyBac transposases but is used to excise IESs in the ciliate Paramecium.

Sounds great, right? Two ciliates with weird genome reduction cycles, both of which use a transposase to remove huge portions of their genomes to make the macronucleus. Except they use different kinds of transposases to do the same job. That's an astonishing convergence.

Or is it?

Baudry et al. 2009. PiggyMac, a domesticated piggyBac transposase involved in programmed genome rearrangements in the ciliate Paramecium tetraurelia. Genes & Development 23(21):2478-2483.