Using the polymerase chain reaction and standard recombinant DNA techniques, a series of new multipurpose low-copy-number (lcn) vectors, pWSK29, pWKS30, pWKS129 and pWKS130, have been constructed. Plasmids pWSK29 and pWKS30 carry the ampicillin-resistance marker (ApR), 20 unique cloning sites flanked by T7 and T3 RNA polymerase promoters, the lacZ alpha gene and the bacteriophage f1 origin of replication (ori) for production of single-stranded (ss) DNA in the presence of a helper phage. Plasmids pWSK129 and pWKS130 carry the kanamycin-resistance marker (KmR) and have 16 unique cloning sites flanked by T7 and T3 RNA polymerase promoters positioned within the lacZ alpha gene. Plasmids pWSK129 and pWKS130 also contain the f1 ori for the generation of ss DNA in the presence of a helper phage. All of the plasmids have an lcn of six to eight per cell. Each vector can be used for: (i) complementation analysis, (ii) generating unidirectional deletions with exonuclease III/S1 nuclease, (iii) DNA sequencing, (iv) high-level gene expression using T7 RNA polymerase, and (v) run-off transcription. They are very useful for analyzing genes encoding proteins which are toxic in Escherichia coli in high copy number.