An improved vector upon introduction into a suitable host containing the thermolabile repressor C.sub.I renders the host capable of effecting expression of a desired gene. The vector is a double-stranded DNA molecule which includes in 5' to 3' order the following: the promoter and operator P.sub.L O.sub.L from lambda bacteriophage; the N utilization site; a first restriction enzyme site permitting replacement of the ribosomal binding site which follows thereafter; a ribosomal binding site; an ATG initiation codon or DNA which is converted into an ATG initiation codon upon insertion of the desired gene into the vector; a second restriction enzyme site for inserting the gene in phase with the ATG codon; a T.sub.1 T.sub.2 rRNA transcription termination sequence; an origin of replication and a gene associated with a selectable or identificable phenotypic trait manifested when the vector is present in the host. The distance between the 3' end of the P.sub.L O.sub.L promoter and operator sequence and the 5' end of the N utilization site is less than about 80 base pairs and the distance between the 3' end of the N utilization site and the 5' end of the ribosomal binding site is less than about 300 base pairs.PPPlasmids have been constructed from the vectors and used to produce bovine, chicken and porcine growth hormones, human apolipoprotein E and human superoxide dismutase.
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