We isolated a replication-thermosensitive mutant of the broad-host-range replicon pVWVOl. The mutant pVE6002 is fuly thermosensitive above 35°C in both gram-negative and gram-positive bacteria. Four clustered mutations were identified in the gene encoding the replication protein of pVE6002. The thermosensitive derivative of the related plasmid pE194 carries a mutation in the analogous region but not in the same position. Derivatives of the thermosensitive plasmid convenient for cloning purposes have been constructed. The low shut-off temperature of pVE6002 makes it a useful suicide vector for bacteria which are limited in their own temperature growth range. Using pVE6002 as the delivery vector for a transposon TnlO derivative in BaciUus subtilis, we observed transposition frequencies of about 1%.

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Lactococcus lactis, a gram-positive organism widely used in the food industry, is a potential candidate for the secretion of biologically useful proteins. We examined the secretion efficiency and capacity of L. lactis by using the Staphylococcus aureus nuclease (Nuc) as a heterologous model protein. When expressed in L. lactis from an efficient lactococcal promoter and its native signal peptide, only ∼60% of total Nuc was present in a secreted form at ∼5 mg per liter. The remaining 40% was found in a cell-associated precursor form. The secretion efficiency was reduced further to ∼30% by the deletion of 17 residues of the Nuc native propeptide (resulting in NucT). We identified a modification which improved secretion efficiency of both native Nuc and NucT. A 9-residue synthetic propeptide, LEISSTCDA, which adds two negative charges at the +2 and +8 positions, was fused immediately after the signal peptide cleavage site. In the case of Nuc, secretion efficiency was increased to ∼80% by LEISSTCDA insertion without altering the signal peptide cleavage site, and the yield was increased two- to fourfold (up to ∼20 mg per liter). The improvement of NucT secretion efficiency was even more marked and rose from 30 to 90%. Similarly, the secretion efficiency of a third protein, the α-amylase ofBacillus stearothermophilus, was also improved by LEISSTCDA. These data indicate that the LEISSTCDA synthetic propeptide improves secretion of different heterologous proteins in L. lactis.

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