The expression of Drosophila melanogaster Hox gene Ultrabithorax is not overtly regulated by the intronic long non-coding RNA lncRNA: PS4 in a wild type genetic background

Long non-coding RNAs (lncRNAs) have been implicated in a variety of processes in development, differentiation, and disease. In Drosophila melanogaster, the bithorax Hox cluster (BX-C) contains three Hox genes (Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B)), along with a number of lncRNAs, most with unknown functions. Here, we investigated the function of a long non-coding RNA, lncRNA: PS4 that originates in the second intron of Ubx and is transcribed in the antisense orientation to Ubx. The expression pattern of lncRNA: PS4 is complementary to Ubx in the thoracic primordia, and the lncRNA: PS4 coding region overlaps the location of the large insertion that causes the dominant homeotic mutation Contrabithorax-1 (UbxCbx-1), which partially transforms Drosophila wings into halteres via ectopic activation of Ubx. This led us to investigate the potential role of this lncRNA in regulation of Ubx expression. The UbxCbx-1 mutation dramatically changes the pattern of lncRNA: PS4, eliminating the expression of most lncRNA: PS4 sequences from parasegment 4 (where Ubx protein is normally absent) and ectopically activating lncRNA: PS4 at high levels in the abdomen (where Ubx is normally expressed). These changes, however, did not lead to changes in the Ubx embryonic transcription pattern. Targeted deletion of the two promoters of lncRNA: PS4 did not result in the change of Ubx expression in the embryos. In the genetic background of a UbxCbx-1 mutation, the lncRNA: PS4 mutation does slightly enhance the ectopic activation of Ubx protein expression in wing discs and also slightly enhances the wing phenotype seen in UbxCbx-1 heterozygotes.