The effects of 3' deletions of the coding and flanking regions of the human tRNAimet gene on its transcription and subsequent processing have been studied both in vitro and in vivo. We demonstrate that in the absence of the oligo T stop signal, polymerase III will read-through efficiently to the next available downstream stop signal. In mutations preserving the 3' terminal sequence of the coding region these read-through transcripts are efficiently processed, irrespective of their length and sequence by an endonucleolytic cleavage to yield both a mature tRNA and an intact trailer RNA. However, deletions involving the terminal regions up to +62 in the coding sequence produce an unprocessed co-transcript of tRNA and downstream sequences. Deletions further within the B promoter box abolish transcription. The use of these mutants as possible "portable" promoters is discussed.