RNase AM, a 5′ to 3′ exonuclease, matures the 5′ end of all three ribosomal RNAs in E. coli

Bacterial ribosomal RNAs (rRNAs) are transcribed as precursors and require processing by Ribonucleases (RNases) to generate mature and functional rRNAs. Although the initial steps of rRNA processing in Escherichia coli (E. coli) were described several decades ago, the enzymes responsible for the final steps of 5S and 23S rRNA 5′-end maturation have remained unknown. Here, I show that RNase AM, a recently identified 5′ to 3′ exonuclease, performs the last step of 5S rRNA 5′-end maturation. RNase AM was also found to generate the mature 5′ end of 23S rRNA, subsequent to a newly identified prior processing step. Additionally, RNase AM was found to mature the 5′ end of 16S rRNA, a reaction previously attributed to RNase G. These findings indicate a major role for RNase AM in cellular RNA metabolism and establish a biological role for the first 5′ to 3′ RNA exonuclease identified in E. coli.