We investigated the cis-acting sequences involved in termination of vesicular stomatis virus mRNA synthesis by using bicistronic genomic analogs. All of the cis-acting signals necessary for termination reside within the first 13 nucleotides of the 23-nucleotide conserved gene junction. This 13- nucleotide termination sequence at the end of the upstream gene comprises the tetranucleotide AUAC, the tract containing seven uridines (U7 tract), and the intergenic dinucleotide (GA), but it does not include the downstream gene start sequence. Data presented here show that upstream mRNA termination is independent of downstream mRNA initiation. Alteration of any nucleotide in the 13-nucleotide sequence decreased the termination activity of the gene junction and resulted in increased synthesis of a bicistronic readthrough RNA. This finding indicated that the wild-type gene junction has evolved to achieve the maximum termination efficiency. The most critical position of the AUAC sequence was the C, which could not be altered without complete loss of mRNA termination. Reducing the length of the wild-type U7 tract to zero, five, or six U residues also totally abolished mRNA termination, resulting in exclusive synthesis of the bicistronic readthrough mRNA. Shortening the wild- type U7 tract to either five or six U residues abolished VSV polymerase slippage during readthrough RNA synthesis. Since neither the U5 nor U6 template was able to direct mRNA termination, these data imply that polymerase slippage is a prerequisite for termination. Evidence is also presented to show that in addition to causing polymerase slippage, the U7 tract itself or its poly(A) product constitutes an essential signal for mRNA termination.