Vectors for high conditional expression of cloned genes.

Several multicopy plasmids carrying the control region of bacteriophage lambda lysogeny, including the gene of a thermosensitive repressor CI857 have been constructed. The phages allow high expression of proteins under the transcription control of lambda promoter PR and translation control of Cro. The method has been assayed by measuring expression of either intact beta-galactosidase, truncated beta-galactosidase or beta-galactosidase fused to a mitochondrial gene product. It is shown that use of a strain with low endogeneous proteolytic activities strongly improves the conditional yield of foreign proteins, so that a high overproduction can be achieved (10-20 per cent of the total protein content of the cell).