SAM and the Anti-Antiterminator

Science's STKE  01 Apr 2003:
Vol. 2003, Issue 176, pp. tw125-TW125
DOI: 10.1126/stke.2003.176.tw125

Gene expression can be controlled by RNA structure, which can affect RNA translation or transcription. In bacteria, direct binding of cellular factors to the nascent RNA, which alters secondary structure, can regulate transcription termination. McDaniel et al. showed that transcripts from the S box genes, which are induced by methionine deprivation, bind S-adenosylmethionine (SAM) in vitro and that SAM, but not methionine or related compounds, represses in vitro transcription. Using an RNA-DNA hybrid cleavage assay with DNA oligonucleotides directed against different regions of one S box gene transcript, the authors showed that cleavage patterns were different in the presence and absence of SAM, consistent with SAM inducing a conformational change in the RNA structure. Under methionine replete conditions, the RNA assumes a terminator conformation. When methionine is depleted, an antiterminator structure is formed that permits readthrough of the terminator. The cleavage assays suggest that SAM promotes the formation of a third structure, the anti-antiterminator, which is necessary for the formation of the terminator structure. In Bacillus subtilis, overexpression of SAM synthetase delayed the stimulation of S box gene expression in response to methionine depletion. Regulation of RNA structure and control of RNA transcription termination are emerging as important regulatory mechanisms in prokaryotes.

B. A. M. McDaniel, F. J. Grundy, I. Artsimovitch, T. M. Henkin, Transcription termination control of the S box system: Direct measurement of S-adenosylmethioinine by the leader RNA. Proc. Natl. Acad. Sci. U.S.A. 100, 3083-3088 (2003). [Abstract] [Full Text]