In neurons, a limited number of mRNAs have been identified in dendritic processes, whereas other transcripts are restricted to the cell soma. Here we have investigated the molecular mechanisms underlying extrasomatic localization of mRNAs encoding microtubule-associated protein 2 (MAP2) in primary neuronal cultures. Vectors expressing recombinant mRNAs were introduced into hippocampal and sympathetic neurons using DNA transfection and microinjection protocols, respectively. Chimeric mRNAs containing the entire 3Ј untranslated region of MAP2 transcripts fused to a nondendritic reporter mRNA are detected in dendrites. In contrast, RNAs containing MAP2 coding and 5Ј untranslated regions or tubulin sequences are restricted to the cell soma. Moreover, 640 nucleotides from the MAP2 3Ј untranslated region (UTR) are both sufficient and essential for extrasomatic localization of chimeric mRNAs in hippocampal and sympathetic neurons. Thus, a cis-acting dendritic targeting element that is effective in two distinct neuronal cell types is contained in the 3Ј UTR of MAP2 transcripts. The observation of RNA granules in dendrites implies that extrasomatic transcripts seem to assemble into multimolecular complexes that may function as transport units.

In mammalian neurons a selected group of mRNAs, including the transcript encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II, is found in dendrites. The molecular mechanisms underlying extrasomatic RNA trafficking are not well described. It is thought that dendritic transcripts contain cis-acting elements that direct their selective subcellular sorting. Here we report the identification of an extrasomatic targeting element in the 3' untranslated region of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II. In primary hippocampal neurons, this 1200-nucleotide-spanning, cis-acting element is sufficient to mediate dendritic localization of chimeric reporter transcripts. The trafficking signal does not share any striking sequence similarity with a previously characterized dendritic targeting element in transcripts encoding the microtubule-associated protein 2. In dendrites of transfected primary neurons, recombinant RNAs form granules with an average diameter of 0.45 microm that may represent preferential RNA docking sites or multimolecular transport units. These findings imply that extrasomatic sorting of individual dendritic mRNAs involves at least partially distinct molecular mechanisms, as well as large trafficking complexes.

Four monoclonal antibodies (MAbs) recognizing HIV-1 reverse transcriptase (RT) were shown here to crossreact with the L LP P subunit of Escherichia coli RNA polymerase (RNAP). The anti-RT MAbs bind to a peptide comprising residues 294^305 of the RT amino acid sequence. Computer analyses revealed sequence similarity between this peptide and two regions of the RNAP L LP P subunit. MAb-binding studies using RT mutants suggested that the epitope is located to amino acids 652^663 of the L LP P sequence. One of the MAbs which inhibited the polymerase activity of RT also mediated a dose dependent inhibition of the RNAP activity. ß

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CorrigendumCorrigendum to: Structural and functional similarities between HIV-1 reverse transcriptase and the Escherichia coli RNA polymerase L LP P subunit (FEBS 24219) [FEBS Letters 484 (2000) 43^47] C