I have determined the nucleotide sequence of the ampicillin resistance gene of pBR322, an Escherichia coli plasmid that encodes a penicillin â-lactamase.  This gene codes for a protein of 286 amino acid residues.  The first 23 amino acids presumably form a signal for secretion, because they do not appear in the mature enzyme, whose partial amino acid sequence has been determined independently, [Ambler, R. P. & Scott, G. K. (1978) Proc. NatL Acad Sci. USA 75, 3732-3736].