Summary: The gene, encoding signal peptidase I (SPase I) was provided with transcription/translation signals and expressed in this organism. When present on a low-copy-number plasmid, the amount of SPase I produced (per mg cell protein) in was half that produced in wild-type cells. The production of SPase I in was increased approximately fivefold by cloning the gene into a high-copy-number plasmid. The expression of SPase I in did not appear to increase the rate of processing of two hybrid secretory precursor proteins. Two observations may explain the failure of SPase I to stimulate processing of exported proteins in . First, the SPase I was apparently not exposed on the outside of the cytoplasmic membrane, indicating its incorrect insertion into the membrane. Second, processing studies, using cell-free extracts of producing SPase I, suggested that the enzyme was not active. A further outcome of this study was that conditions favouring processing of precursors by SPase I in cell-free extracts of did not favour processing by the corresponding enzyme in cell-free extracts. This suggests that significant differences exist between the two enzymes. The observation that antibodies directed against SPase I did not cross-react with membrane proteins supports this idea.


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