Practice question

S-adenosylmethionine riboswitches constitute widespread SAM-I, SAM-II, SAM-III families positioned upstream of genes encoding enzymes for sulfur assimilation, methionine biosynthesis, SAM synthetase metK, and methionine importers. When intracellular SAM abundant, binding to aptamer core favors terminator stem or SD sequestration, repressing transcription or translation and preventing unnecessary synthesis. Low SAM shifts equilibrium toward antiterminator and ribosome access, inducing de novo methionine production from homoserine, cysteine, and methyl-tetrahydrofolate precursors. This feedback loop maintains methyl donor homeostasis vital for methylation of DNA, RNA, proteins, polyamine synthesis, and one-carbon metabolism across many Gram-positive bacteria and archaea.