Translational repressorbruno plays multiple roles in development and is widely conserved

Phylogenetic and Expression Analysis of RNA-binding Proteins with Triple RNA Recognition Motifs in Plants

“…Bruno proteins are well conserved in multi-cellular organisms and act as translational repressors that play a key role during gametogenesis (Webster et al, 1997). Clade 4 RBPs had a motif organization that was also identified among nucleolins important for ribosome biogenesis (Srivastava and Schlessinger, 1990).…”

Dual roles of p82, the clam CPEB homolog, in cytoplasmic polyadenylation and translational masking

“…The growing body of evidence, gathered from studies of RNA stability and translation showing that the 59 and 39 ends of the mRNA interact (Caponigro & Parker, 1995;Tarun & Sachs, 1996;Gunkel et al+, 1998;Preiss & Hentze, 1998), has led to models in which 39 UTR binding repressors interfere with this interaction and prevent contacts necessary for initiation+ Recent studies suggest that such contacts do not necessarily involve the 59 cap structure or cap-binding factors+ hnRNP K and E, which bind a pyrimidine-rich repeat motif in lipoxygenase 39 UTR, inhibit translation initiation at the level of 80S ribosome formation+ Furthermore, since they control both cap-dependent and internal ribosome entry site-mediated translation in vitro, these proteins exert their repression at a step downstream of the convergence of the two translation initiation systems, that is ribosome assembly at the AUG codon (Ostareck et al+, 1997)+ Interestingly, Nanos and Pumilio, which collaborate to regulate the translation of maternal hunchback mRNA, can also repress cap-independent translation from an IRES in vivo (Wharton et al+, 1998)+ Whether this model of inhibition of translation will be applicable to other 39 UTR repressors (e+g+, this work; Webster et al+, 1997;Culp & Musci, 1998) remains to be seen+ In any case, since no specific developmental translational repressor has yet been cloned that is capable of reconstituting repression in vitro, it may be necessary to invoke the possibility that such repressors exert their effects through or in collaboration with other factors+ How can both functions of clam p82/CPEB be reconciled? In other words, how does the same protein act both as a repressor and as an activator of mRNA expression?…”

Smaug, a novel and conserved protein, contributes to repression of nanos mRNA translation in vitro

“…The sequence of Smg provides no obvious suggestion of how it may act in translational repression as a consequence of binding to the nos 39 UTR+ This is not surprising, as very few translational repressors have been characterized and the only domains whose function is understood are those involved in RNA binding (for examples, see Murata & Wharton, 1995;Dubnau & Struhl, 1996;Rivera-Pomar et al+, 1996;Bashaw & Baker, 1997;Ostareck et al+, 1997;Webster et al+, 1997;Zhang et al+, 1997;Wharton et al+, 1998;Jan et al+, 1999)+ At present, nos is the only known transcript with a recognizable SRE, despite sequence inspection of numerous other maternally expressed genes+ Neverthe-less, there may well be other targets, as the ubiquitous appearance of Smg in early embryos should allow it to act on other mRNAs+ In addition, Smg may also act later in development, when nos mRNA is no longer present+ Although by Western blot analysis we can only detect Smg protein in early embryos, this method might not reveal the expression of Smg in small populations of cells at the later stages+ Indeed, whole-mount antibody stains suggest that the Smg protein appears in the region of the ventral nerve cord as well as the brain during embryogenesis (C+A+ Smibert, Y+S+ Lie, W+ Shillinglaw, W+J+ Henzel, P+M+ Macdonald, unpubl+ data), and a smg EST cDNA clone has been identified in a Drosophila head cDNA library (BDGP; GenBank accession number AI134156)+ Identification of potential target mRNAs, those that contain SRE-like sequences, will soon be possible on a genome-wide scale when sequencing of the genome is complete+…”