Blood from 'junk': the LTR chimeric transcript Pu.2 promotes erythropoiesis

Transposable elements (TEs) are a prominent feature of most eukaryotic genomes. Despite rapidly accumulating evidence for the role of TE-driven insertional mutagenesis and structural variation in genome evolution, few clear examples of individual TEs impacting biology via perturbed gene regulation are available. A recent report describes the discovery of an alternative promoter for the murine erythroid transcription factor Pu.1. This promoter is located in an ORR1A0 long terminal repeat (LTR) retrotransposon intronic to Pu.1 and is regulated by the Krüppel-like factors KLF1 and KLF3. Expression of the resultant chimeric transcript, called Pu.2, spontaneously induces erythroid differentiation in vitro. These experiments illustrate how transcription factor binding sites spread by retrotransposition have the potential to impact networks encoding key biological processes in the host genome.