In both Drosophila and mammals, IB kinases (IKKs) regulate the activity of Rel/NF-B transcription factors by targeting their inhibitory partner proteins, IBs, for degradation. We identified mutations in ik2, the gene that encodes one of two Drosophila IKKs, and found that the gene is essential for viability. During oogenesis, ik2 is required in an NF-B-independent process that is essential for the localization of oskar and gurken mRNAs; as a result, females that lack ik2 in the germline produce embryos that are both bicaudal and ventralized. The abnormal RNA localization in ik2 mutant oocytes can be attributed to defects in the organization of microtubule minus-ends. In addition, both mutant oocytes and mutant escaper adults have abnormalities in the organization of the actin cytoskeleton. These data suggest that this IB kinase has an NF-B-independent role in mRNA localization and helps to link microtubule minus-ends to the oocyte cortex, a novel function of the IKK family.

Nucleus-encoded proteins interact with cis-acting elements in chloroplast transcripts to promote RNA stability and translation. We have analyzed the structure and function of three such elements within the Chlamydomonas petD 5 untranslated region; petD encodes subunit IV of the cytochrome b 6 /f complex. These elements were delineated by linker-scanning mutagenesis, and RNA secondary structures were investigated by mapping nuclease-sensitive sites in vitro and by in vivo dimethyl sulfate RNA modification. Element I spans a maximum of 8 nucleotides (nt) at the 5 end of the mRNA; it is essential for RNA stability and plays a role in translation. This element appears to form a small stem-loop that may interact with a previously described nucleus-encoded factor to block 533 exoribonucleolytic degradation. Elements II and III, located in the center and near the 3 end of the 5 untranslated region, respectively, are essential for translation, but mutations in these elements do not affect mRNA stability. Element II is a maximum of 16 nt in length, does not form an obvious secondary structure, and appears to bind proteins that protect it from dimethyl sulfate modification. Element III spans a maximum of 14 nt and appears to form a stem-loop in vivo, based on dimethyl sulfate modification and the sequences of intragenic suppressors of element III mutations. Furthermore, mutations in element II result in changes in the RNA structure near element III, consistent with a long-range interaction that may promote translation.