Induction by Mercuric Ion of Extensive Degradation of Cellular Ribonucleic Acid in Escherichia coli

Abstract

Low concentrations of HgCl 2 were found to induce extensive degradation of ribonucleic acid (RNA) in exponentially growing Escherichia coli cells but not in stationary-phase cells. Whereas 80% of cellular RNA was degraded during 90 min of incubation with 10 −5 m HgCl 2 at 37 C, HgCl 2 caused only slight degradation in stationary cells, even when present at concentrations higher than 5 × 10 −5 m . Inhibition of RNA synthesis occurred at almost the same concentration of HgCl 2 as degradation, and the ability of stationary-phase cells to synthesize RNA was also resistant to HgCl 2 . The transition of cells from complete sensitivity to HgCl 2 to a fully insensitive state took place simultaneously with the cessation of growth. p -Chloromercuribenzoate was also found to induce remarkable degradation of RNA. In E. coli Q13, a mutant deficient for ribonuclease I, no degradation of RNA was evident, even in the exponential growth phase. 3′-Mononucleotides but not 5′-mononucleotides were found among the degradation products of cellular RNA. 2′,3′-Cyclic mononucleotides were produced when RNA was degraded by the cell-free extracts of the Hg treated cells. Almost complete unmasking of the latent ribonuclease occurred in the particle fraction containing subribosomal particles of the Hg-treated cells. These data suggest that the incubation of exponentially growing E. coli cells with HgCl 2 led to the unmasking of ribonuclease I, which resulted in the extensive degradation of cellular RNA. The activation of ribonuclease by HgCl 2 in the isolated particulate fraction of E. coli K-12 which occurred in vitro suggested the presence of an Hg-sensitive inhibitor for ribonuclease I.