Analysis of lambda receptor and beta-lactamase synthesis and export using cloned genes in a minicell system

We have cloned lamB, the gene for lambda receptor (an outer membrane protein), on a small plasmid which also carries the gene for beta-lactamase (a periplasmic protein). We have identified a promoter in the region of malK, the gene immediately preceding lamB, which is active in minicells but relatively inactive in vitro. Using a minicell system, we have found that both lambda receptor and beta-lactamase are made as full length precursors which are subsequently processed. We also show that the lambda receptor precursor can be exported to the outer membrane before it is processed. Mature beta-lactamase is found only in the periplasm, suggesting that processing may be a requirement for export to the periplasm.