Use of gene fusions to determine a partial signal sequence of alkaline phosphatase

We have isolated strains of Escherichia coli in which an amino-terminal portion of the cytoplasmic enzyme beta-galactosidase is replaced by an amino-terminal portion of the periplasmic enzyme alkaline phosphatase. The synthesis of these hybrid proteins is regulated by inorganic phosphate and they are located in the cytoplasm. One of these proteins was purified, and 14 amino acids of the amino-terminal sequence were determined. The first five amino acids, Met-Lys-Gln-Ser-Thr, appear to represent a portion of the signal sequence of the precursor of alkaline phosphatase, and the remaining sequence corresponds to that of beta-galactosidase, beginning at amino acid residue 20. The approach described here could be used for the analysis of signal sequences of exported proteins and for partial amino acid sequence determination of certain of certain other proteins.