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Adenosylcobalamin Inhibits Ribosome Binding to btuB RNA

Xiangwu Nou and Robert J. Kadner
Proceedings of the National Academy of Sciences of the United States of America
Vol. 97, No. 13 (Jun. 20, 2000), pp. 7190-7195
Stable URL: http://www.jstor.org/stable/122789
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Adenosylcobalamin Inhibits Ribosome Binding to btuB RNA
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Abstract

Expression of the btuB gene encoding the outer membrane cobalamin transporter in Escherichia coli is strongly reduced on growth with cobalamins. Previous studies have shown that this regulation occurs in response to adenosylcobalamin (Ado-Cbl) and operates primarily at the translational level. Changes in the level and stability of btuB RNA are consequences of the modulated translation initiation. To examine how Ado-Cbl affects translation, the binding of E. coli 30S ribosomal subunits to btuB RNA was investigated by using a primer extension inhibition assay. Ribosome binding to btuB RNA was much less efficient than to other RNAs and was preferentially lost when the ribosomes were subjected to a high-salt wash. Ribosome binding to btuB RNA was inhibited by Ado-Cbl but not by cyanocobalamin, with half-maximal inhibition around 0.3 μ M Ado-Cbl. Ribosome-binding activity was increased or decreased by mutations in the btuB leader region, which affected two predicted RNA hairpins and altered expression of btuB-lacZ reporters. Finally, the presence of Ado-Cbl elicited formation of a single primer extension-inhibition product with the same specificity and Cbl-concentration dependence as the inhibition of ribosome binding. These results indicate that btuB expression is controlled by the specific binding of Ado-Cbl to btuB RNA, which then affects access to its ribosome-binding sequence.

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