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Figure 1. Repression by non-coding (nc) RNA is essential to generate a pattern of Ultrabithorax expression in early Drosophila embryos.

Top: Time-course of nascent Ubx and ncRNA transcription. Nascent ncRNA was detected with probes indicated specifc to ncRNA (red) and the coding Ubx HOX gene (green). At syncytial blastoderm, ncRNA is expressed earlier than Ubx (left panels). The initial expression domain of Ubx is anterior to that of ncRNA (middle panels, including blow-up of merge). During germband elongation, expression domains of ncRNA in each parasegment are anterior to those of Ubx (right panels).

Bottom: A model for the role of the Trithorax-containing TAC1 in transcription. TAC1 recruitment along with elongation factors and histone modification is necessary for efficient transcriptional elongation of Ubx and ncRNAs in complementary cells in the posterior and anterior regions of each parasegment, respectively. Efficient elongation of Pol II from promoters of ncRNAs proceeds through the 5’-regulatory elements of Ubx, thus preventing its expression. This generates a mosaic pattern of Ubx expression within embryonic parasegments (right).

Petruk, S., Sedkov, Y., Riley, K.M., Hodgson, J., Schweisguth, F., Hirose, S., Jaynes, J.B., Brock, H.W. and Mazo, A. (2006). Transcription of bxd non-coding RNAs promoted by Trithorax represses Ubx in cis by transcriptional interference. Cell, 127, 1209-1221.