Abstract: 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 identifiable phenotypic trait manifested when the vector is present in the host. The distance between the 3' end of the P.sub.L O.sub.