A bidirectional rho-independent transcription terminator between the E. coli tonB gene and an opposing gene

We identified an Escherichia coli gene, designated P14, that is adjacent to and in the opposite orientation to the tonB gene. The 36 base pair intercistronic region between tonB and P14 contains a novel rho-independent transcription terminator that functions bidirectionally, both in vivo and in vitro, to terminate tonB and P14 transcription. Transcription of tonB and P14 terminates at symmetrically equivalent nucleotides, such that the 3' ends of tonB and P14 transcripts are complementary. The terminator is 70% efficient in both directions in vitro. Interestingly, relative rates of in vivo RNA synthesis, immediately prior to and following the terminator, appear to indicate that it is more efficient in the tonB direction (95%) than in the P14 direction (70%). We discuss the possibility that this gene arrangement has regulatory consequences for the expression of tonB.