Cutting in-line with iron: ribosomal function and non-oxidative RNA cleavage

Divalent metal cations are essential to the structure and function of the ribosome. Previous characterizations of the ribosome performed under standard laboratory conditions have implicated Mg 2+ as a primary mediator of ribosomal structure and function. Possible contributions of Fe 2+ as a ribosomal cofactor have been largely overlooked, despite the ribosome’s early evolution in a high Fe 2+ environment, and its continued use by obligate anaerobes inhabiting high Fe 2+ niches. Here we show that (i) Fe 2+ cleaves RNA by in-line cleavage, a non-oxidative mechanism that has not previously been shown experimentally for this metal, (ii) the first-order rate constant with respect to divalent cations is more than 200 times greater with Fe 2+ than with Mg 2+ , (iii) functional ribosomes are associated with Fe 2+ after purification from cells grown under low O 2 and high Fe 2+ , and (iv) a small fraction of Fe 2+ that is associated with the ribosome is not exchangeable with surrounding divalent cations, presumably because it is tightly coordinated by rRNA and buried in the ribosome. In total, these results expand the ancient role of iron in biochemistry and highlight a possible new mechanism of iron toxicity. Key Points Fe 2+ cleaves rRNA by a non-oxidative in-line cleavage mechanism that is more than 200 times faster than in-line cleavage with Mg 2+ ; ribosomes purified from cells grown under low O 2 and high Fe 2+ retain ~10 Fe 2+ ions per ribosome and produce as much protein as low O 2 , high Mg 2+ -grown ribosomes; a small fraction (~2%) of Fe 2+ that is associated with the ribosome is not exchangeable.