Characterization of a new rho mutation that relieves polarity of Mu insertions.

We report the identification and characterization of a new rho mutation, rho614, that relieves polarity of Mu insertions in Escherichia coli. The mutation was identified by its ability to suppress the polarity of the Mu-mediated phi(lamB'-'lacZ)hyb61-4 fusion that is located at codon four of the lamB signal sequence. The rho614 mutation alters residue 80 in the proposed RNA-binding domain of Rho and is recessive to wild-type rho. We suggest that in the presence of the rho614 allele transcripts initiated at the Mu promoter PcM fail to terminate at presumptive Rho-dependent termination sites, namely rut1 and rut2, and continue into the 3' 'lamB gene allowing a LamB+ phenotype. This contention is supported by deletion analysis of the region and the observation that insertional inactivation of genes that reduce transcription from PcM, clpP (ATP protease), himA (IHF-alpha), and himD (IHF-beta), block the LamB+ phenotype. rho614, rho4 and rho201 alleles suppress the polarity of a malK::Mu insertion on the downstream lamB gene. However, the polarity of the phi(lamB'-'lacZ)hyb61-4 insertion is only suppressed by the rho614 mutation. We propose that the rho614 mutation allows suppression of transcriptional polarity without interfering with translation initiation signals of the truncated 'lamB gene. In addition to identifying a new rho mutation and Rho-dependent terminator sequence, this system provides a means of studying Rho protein/terminator relationships through the identification of new classes of rho mutations.