Functional Validation of SAM Riboswitch Element A from Listeria monocytogenes

SreA is one of seven candidate S-adenosyl methionine (SAM) class I riboswitches identified in Listeria monocytogenes, a saprophyte and opportunistic foodborne pathogen. SAM is essential to all domains of life, serving as a ubiquitous methyl donor and mediator of trans-sulfuration. SreA precedes genes encoding a methionine ATP-binding cassette (ABC) transporter, which imports methionine, a sulfur containing amino acid and substrate for sulfur metabolism. SreA is presumed to regulate transcription of its downstream genes in a SAM-dependent manner. The proposed role of SreA in controlling the transcription of genes encoding an ABC transporter complex may have important implications for how the bacteria senses and responds to the availability of the metabolite SAM in the diverse environments in which L. monocytogenes persists. Here we validate SreA as a functional SAM-I riboswitch through ligand binding studies, structure characterization, and transcription termination assays. We determined that SreA has both a similar structure and SAM binding properties to other well characterized SAM-I riboswitches. Interestingly, SreA regulates transcription at a distinctly lower (nM) ligand concentration than other SAM riboswitches but does not substantially terminate transcription, even in the presence of mM SAM. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=101 SRC="FIGDIR/small/596223v1_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@16b3af4org.highwire.dtl.DTLVardef@eb6aecorg.highwire.dtl.DTLVardef@1b8e7ecorg.highwire.dtl.DTLVardef@823cf7_HPS_FORMAT_FIGEXP M_FIG C_FIG