Some of the spatial cues which direct early patterning events in Drosophila embryogenesis are maternal mRNAs localized in the oocyte during oogenesis. For example, Bicaudal-D, fs (1) K10, and orb RNAs are transiently localized at the anterior oocyte margin in mid oogenesis, and oskar RNA is localized at the posterior oocyte margin beginning in mid oogenesis. Using inhibitors of cytoskeletal function, we find that microtubules, but not microfilaments, are required for localization of these mRNAs during oogenesis, results similar to those described earlier for bicoid RNA. However, the RNAs show a differential sensitivity to microtubule inhibitors. Anterior localization of Bicaudal-D, fs (1) K10, and orb RNAs is completely disrupted following even mild drug treatments. bicoid RNA localization is intermediate in its response to microtubule drugs, while oskar RNA localization is much more resistant. In addition, the localized RNAs respond differently to taxol, a microtubule stabilizing agent. The differences among these RNAs suggest that factors other than microtubules are required to maintain the positions of localized RNAs in the oocyte. Microtubules are also required for the preferential accumulation of these transcripts in the previtellogenic oocyte, consistent with the idea that these mRNAs are transported by a microtubule-dependent mechanism to the oocyte.
PTL-1, a microtubule-associated protein of the structural MAP2/tau family, is the sole member of this gene family in Caenorhabditis elegans. Sequence analysis of available invertebrate genomes revealed a number of single, putative tau-like genes with high similarity to ptl-1. The ptl-1 gene is expressed in a number of cells, most notably mechanosensory neurons. We examined the role of ptl-1 in Caenorhabditis elegans in adult neurons as well as during development. A ptl-1 knockout strain of worms exhibited an egg hatching defect, as well as a reduced sensitivity to touch stimuli. In addition, the knockout allele ptl-1(ok621) acts as a dominant enhancer of several temperaturesensitive alleles of mec-7 and mec-12, which code the isoforms of β-tubulin and α-tubulin that together form the unusual 15 protofilament microtubules involved in touch sensation. These results demonstrate for the first time a functional role for this microtubule-associated protein in nematodes and suggest that PTL-1 is involved in mechanosensation as well as some aspect of embryogenesis.
Different classes of RNAs are exported from Xenopus laevis oocyte nuclei by facilitated pathways. We have performed kinetic competition analyses to investigate the relationship between the export pathways of microinjected tRNA and ribosomal subunits. Saturating concentrations of ribosomal subunits do not compete tRNA export. Thus, the saturable factor in the ribosomal subunit export pathway is not limiting for tRNA export. The co-microinjection of ribosomal subunits did, however, stimulate the rate of tRNA export. Co-injected mRNA also stimulated tRNA export. tRNA export itself displays positive cooperative export kinetics that are abrogated by saturating concentrations of rRNA. These results are consistent with the existence of common high affinity RNA-binding sites that can be titrated with tRNA, rRNA or ribosomal subunits, and mRNA. Furthermore, high concentrations of tRNA are also shown to have moderate inhibitory effects on 40 S subunit export, indicating a lower affinity common intermediate also shared by mRNA.
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