In the course of characterizing the bacteriocin lactocin S and its encoding gene, we discovered three alanine-for-serine substitutions which, apparently, is a violation of the genetic code. Subsequent chiral analysis of lactocin S hydrolysates revealed a correlation between D-alanine content and the three substitutions, implying a conversion of L-serine to D-alanine in lactocin S maturation. In order to explain this observation, we suggest a sequence of events initiated by the dehydration of serine, which is common in the biosynthesis of the lanthionine-containing polycyclic lantibiotics (Schnell, N., Entian, K.-D., Schneider, U., Götz, F., Zähner, H., Kellner, R. & Jung, G. (1988) Nature 333, 276-278; Jung, G. (1991) Angew, Chem. Int. Ed. Engl. 30, 1051-1068; Bierbaum, G. & Sahl, H.-G. (1993) Zentralbl. Bakteriol. 278, 1-22) and completed by the stereospecific reduction of dehydroalanine residues. The occurrence of non-lanthionine alpha-carbon stereoinversion in lactocin S maturation substantiates the hypothetical alpha-epimerization scheme originally put forward by Bycroft (Bycroft, B. W. (1969) Nature 224, 595-597), and we propose a revision of this model to accommodate the lactocin S-type stereoinversion. Lactocin S is the first prokaryotic exception to the rule that only L-amino acids are included in ribosomally synthesized peptides.
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