Enhanced expression of cro-beta-galactosidase fusion proteins under the control of the PR promoter of bacteriophage lambda.

Hybrid plasmids carrying cro-lacZ gene fusions have been constructed by joining DNA segments carrying the PR promoter and the start of the cro gene of bacteriophage lambda to the lacZ gene fragment carried by plasmid pLG400 . Plasmids in which the translational reading frames of the cro and lacZ genes are joined in-register (type I) direct the synthesis of elevated levels of cro-beta-galactosidase fusion protein amounting to 30% of the total cellular protein, while plasmids in which the genes are fused out-of-register (type II) produce a low level of beta-galactosidase protein. Sequence rearrangements downstream of the cro initiator AUG were found to influence the efficiency of translation, and have been correlated with alterations in the RNA secondary structure of the ribosome-binding site. Plasmids which direct the synthesis of high levels of beta-galactosidase are conditionally lethal and can only be propagated when the PR promoter is repressed. Deletion of sequences downstream of the lacZ gene restored viability, indicating that this region of the plasmid encodes a function which inhibits the growth of the cells. The different applications of these plasmids for expression of cloned genes are discussed.