Bruno negatively regulates germ cell-less expression in a BRE-independent manner

Mechanisms of post-transcriptional control are essential during Drosophila oogenesis and embryogenesis to sequester gene products in discrete regions and ultimately achieve embryonic asymmetry. Maternal germ cell-less (gcl) mRNA accumulates in the pole plasm of the embryo before Gcl protein is detectable. gcl mRNA, but not Gcl protein, can also be detected in somatic regions of the embryo, suggesting that gcl RNA is subject to translational control. We find that Gcl is expressed during oogenesis, and that it is regulated by the translational repressor Bruno (Bru). Increased levels of Gcl are observed in the oocyte when Bru level is reduced, and overexpression of Bru reduces Gcl expression. Consistently, reduction of the maternal dosage of Bruno leads to ectopic Gcl expression in the embryo, which, in turn, represses anterior hückebein (hkb) expression. Bru binds directly to the gcl 3’UTR in vitro, but, surprisingly, this binding is independent of a BRE (Bruno response element)-like motif. This motif is also not required for in vivo repression of Gcl expression during oogenesis or early embryogenesis. Bru binds the gcl 3’UTR via its C-terminal domain, which includes RNA recognition motif 3 (RRM3), with little or no contribution from the remainder of the protein. We conclude that repression by Bruno during oogenesis is required to restrict Gcl expression in the early embryo and that Bru represses gcl expression in a manner that involves RRM3 and a sequence unrelated to the BRE.