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Orb, Orb, and More Orb

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Science's STKE  27 Mar 2001:
Vol. 2001, Issue 75, pp. tw9
DOI: 10.1126/stke.2001.75.tw9

In developing Drosophila embryo, pathways that produce distinct accumulations of specific maternal mRNAs control axis determination. One gene that is involved in both the dorsoventral axis and the posterior axis determination is orb, which encodes a protein similar to Xenopus cytoplasmic polyadenylation element-binding protein (CPEB), which is required to activate translation of masked maternal mRNAs. Orb is properly localized before the arrival of mRNAs encoding the axis determinants; and in oocytes from flies with disrupted orb genes, axis determination is abnormal. The localization of Orb mRNA is known to depend on the 3′ untranslated region (UTR). Tan et al. show that the 3′ UTR of Orb can act as a dominant negative when expressed in both the sense and antisense direction. Furthermore, their studies indicate that Orb mRNA localization is dependent on the expression of the Orb protein in a dose-dependent fashion and that Orb protein directly binds Orb mRNA, suggesting that Orb localization is controlled through an autoregulatory positive feedback loop. Thus, Orb mRNA interacts with Orb protein to promote Orb expression, which is then available to bind and activate more Orb mRNA. The chicken and egg question still remains about how the first Orb protein or mRNA gets properly targeted, but this paper provides a mechanism for the accumulation of Orb once the site has been established.

L. Tan, J. S. Chang, A. Costa, P. Schedl, An autoregulatory feedback loop directs the localized expression of the Drosophila CREB protein Orb in the developing oocyte. Development 128, 1159-1169 (2001). [Online Journal]

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