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J. Cell Biol.,
Volume 144, Number 4, February 22, 1999 603-615
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.
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