In Escherichia coli and Bacillus subtilis replication fork arrest occurs in the terminus at sequence-specific sites by the binding of replication terminator proteins to the fork arrest sites. The protein-DNA complex causes polar arrest of the replication forks by inhibiting the activity of the replicative helicases in only one orientation of the terminus with respect to the replication origin. This activity has been named as polar contrahelicase. In this paper we report on a second novel activity of the terminator proteins of E.coli and B.subtilis, namely the ability of the proteins to block RNA chain elongation by several prokaryotic RNA polymerases in a polar mode. The replication terminator proteins ter and RTP of E.coli and B.subtilis respectively, impeded RNA chain elongation catalyzed by T7, SP6 and E.coli RNA polymerases in a polar mode at the replication arrest sites. The RNA chain anti-elongation and the contrahelicase activities were isopolar. Whereas one monomer of ter was necessary and sufficient to block RNA chain elongation, two interacting dimers of RTP were needed to effect the same blockage. The biological significance of the RNA chain anti-elongation activity is manifested in the functional inactivation of a replication arrest site by invasion of RNA chains from outside, and the consequent need to preserve replication arrest activity by restricting the passage of transcription through the terminus-terminator protein complex.