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© The Rockefeller University Press, 0021-9525/1999//603 $5.00
The Journal of Cell Biology, Volume 144, Number 4, , 1999 603-615

Regular Articles

The RNA-editing Enzyme ADAR1 Is Localized to the Nascent Ribonucleoprotein Matrix on Xenopus Lampbrush Chromosomes but Specifically Associates with an Atypical Loop



Christian R. Eckmann and Michael F. Jantsch

Department of Cytology and Genetics, Institute of Botany, University of Vienna, A-1030 Vienna, Austria

Double-stranded RNA adenosine deaminase (ADAR1, dsRAD, DRADA) converts adenosines to inosines in double-stranded RNAs. Few candidate substrates for ADAR1 editing are known at this point and it is not known how substrate recognition is achieved. In some cases editing sites are defined by basepaired regions formed between intronic and exonic sequences, suggesting that the enzyme might function cotranscriptionally. We have isolated two variants of Xenopus laevis ADAR1 for which no editing substrates are currently known. We demonstrate that both variants of the enzyme are associated with transcriptionally active chromosome loops suggesting that the enzyme acts cotranscriptionally. The widespread distribution of the protein along the entire chromosome indicates that ADAR1 associates with the RNP matrix in a substrate-independent manner. Inhibition of splicing, another cotranscriptional process, does not affect the chromosomal localization of ADAR1. Furthermore, we can show that the enzyme is dramatically enriched on a special RNA-containing loop that seems transcriptionally silent. Detailed analysis of this loop suggests that it might represent a site of ADAR1 storage or a site where active RNA editing is taking place. Finally, mutational analysis of ADAR1 demonstrates that a putative Z-DNA binding domain present in ADAR1 is not required for chromosomal targeting of the protein.

Key Words: RNA editing • chromosomal localization • RNA splicing • Xenopus oocytes • epitope tagging

Abbreviations used in this paper: AMD, actinomycin D; dsRNA, double-stranded RNA; dsRBD, double-stranded RNA-binding domain; GV, germinal vesicle; LBC, lampbrush chromosome; RNP, ribonucleoprotein particle; snRNA, small nuclear RNA; SLL, sequentially labeling loop.

Address correspondence to Michael F. Jantsch, Department of Cytology and Genetics, Institute of Botany, University of Vienna, Rennweg 14, A-1030 Vienna, Austria. Tel.: 43 14277 54030. Fax: 43 1 4277 9541. E-mail: jantsch{at}s1.botanik.univie.ac.at


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