Plasmid vectors are described that allow cloning of target DNAs at sites where they will be minimally transcribed by Escherichia coli RNA polymerase but selectively and actively transcribed by T7 RNA polymerase, in vitro or in E. coli cells. Transcription is controlled by the strong phi 10 promoter for T7 RNA polymerase, and in some cases by the T phi transcription terminator. The RNA produced can have as few as two foreign nucleotides ahead of the target sequence or can be cut by RNase III at the end of the target sequence. Target mRNAs can be translated from their own start signals or can be placed under control of start signals for the major capsid protein of T7, with the target coding sequence fused at the start codon or after the 2nd, 11th or 260th codon for the T7 protein. The controlling elements are contained on small DNA fragments that can easily be removed and used to create new expression vectors.