Identification of amino acid sequences that can function as translocators of ?-lactamase in Escherichia coli

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Abstract
A plasmid vector, pYZ1, was constructed which lacks most of the .beta.-lactamase signal-peptide coding region, but has a unique EcoRI site spanning codons 2 and 3 of the resultant cytoplasmic .beta.-lactamase derivative. Short quasi-random DNA sequences were cloned into the EcoRI site and Escherichia coli transformants in which some translocation of .beta.-lactamase across the cytoplasmic membrane was restored were selected by their ability to survive and form colonies on plates containing a low level of ampicillin. About 15-20% of all in-frame inserts restored some .beta.-lactamase translocation and the salient feature of these sequences was their marked hydrophobicity. These results are discussed in the light of a similar study in which sequences able to function as translocators of invertase in yeast were cloned and analysed (Kaiser et al., 1987).