The present invention provides a recombinant transcription unit capable of producing an RNA transcript of a predetermined size comprising a regulatory sequence operably linked to a nucleotide sequence comprising a transcribed region such that the transcription of said transcribed region is controlled by said regulatory sequence. The transcribed region comprises a region that encodes for a viral sequence, and a non-coding region downstream of the region encoding for said viral sequence, wherein the non-coding region comprises a nucleotide sequence encoding a cis-acting ribozyme. Methods of using the recombinant transcription unit, and cells containing vectors comprising the recombinant transcription unit are also disclosed.