Transcriptional pulsing of a nucleolar transgene

The pulsatile nature of transcription has recently emerged as an important property of gene expression. Here we report on the characterization of a RNA polymerase II transgene that is transcribed in the nucleolus. Using the MS2-GFP reporter system and live cell imaging, we found that the synthesis of a MS2-tagged transcript in the nucleolus was discontinuous in all of the cells that were observed, with periods of activity lasting from 15 minutes to 21 hours. The frequency of pulse lengths could be fitted with an exponential function, from which we determined that transcription occurs on average for periods of 20 minutes. These ON periods alternate with periods of inactivity which last on average 29 minutes. The post-mitotic re-activation of transcription was found to be asynchronous in daughter cell pairs. Our observation of discontinuous transcriptional activity in the nucleolus may reflect cycling in the assembly and disassembly of active chromatin structure in and/or around the rDNA genes.