MtrB regulates transcription attenuation of the Bacillus subtilis trp operon. We have shown that MtrB, either from B. subtilis or overexpressed in Escherichia coli, binds specifically to RNA from the leader region of the trp operon by a gel mobility-shift assay. This binding is tryptophan dependent. MtrB binds to a transcript terminated at the trp attenuator (-2 to +138) or a read-through transcript (-2 to +318). MtrB does not bind antisense trp leader RNA or single-stranded trp leader DNA. These results support the model in which attenuation is controlled by tryptophan-activated MtrB influencing the secondary structure of the leader region transcript to form a terminator structure.