Processing of the N termini of nascent polypeptide chains requires deformylation prior to methionine removal

N-formyl-methionine termini are formed in the initiation reaction of bacter ial protein synthesis and processed during elongation of the nascent polype ptide chain. We report that the formyl group must be removed before the met hionine residue can be cleaved by methionine aminopeptidase. This has long been implicity assumed, but that assumption was based on inconclusive data and was in apparent conflict with more recently published data. We demonstr ate that the Salmonella typhimurium methionine aminopeptidase is totally in active on an N-formyl methionyl peptide in vitro, and present a detailed ch aracterization of the substrate specificity of this key enzyme by use of a very sensitive and quantitative assay. Finally, a reporter protein expresss ed in a strain lacking peptide deformylase was shown to retain the formyl g roup confirming the physiological role of the deformylase. (C) 1999 Academi c Press.