Amino‐Acid Sequence of lac Repressor from Escherichia coli

The lac repressor from Escherichia coli , composed of four identical subunits with a molecular weight of 37 160, was carboxymethylated and fragmented by tryptic digestion and cyanogen bromide treatment. Using ion‐exchange chromatography, gel filtration and preparative thin‐layer electrophoresis and chromatography 29 of the 30 tryptic peptides were isolated in pure form. Direct Edman degradation and the dansyl‐Edman technique were used to determine the sequence of the small tryptic peptides. Special emphasis was put on the sequence determination of the six large tryptic fragments which together account for 177 residues, corresponding to 51% of the repressor subunit with its 347 residues. The large tryptic fragments were analyzed after fragmentation with chymotrypsin, thermolysin and dipeptidyl aminopeptidase I. Thus the sequence of all 30 tryptic peptides could be deduced. The complete sequences of all cyanogen bromide fragments were deduced from peptides obtained by tryptic, chymotryptic and thermolytic digestion of the individual fragments and by automated stepwise Edman degradation of lac repressor and of the large cyanogen bromide fragments. The order of the cyanogen bromide fragments was given by overlapping tryptic peptides. The resulting amino acid composition of the monomer is Asp 15 , Asn 11 , Thr 18 , Ser 30 , Glu 14 , Gln 27 , Pro 13 , Gly 22 , Ala 44 , Cys 3 , Val 33 , Met 9 , Ile 17 , Leu 40 , Tyr 8 , Phe 4 , Trp 2 , Lys 11 , His 7 , Arg 19 . The sequence of lac repressor shows no similarities with that of other proteins known to bind to DNA or RNA. The N‐terminal 55 residues contain two homologous regions. This part of the sequence which is involved in lac operator binding might have been formed by gene duplication.