Rho-dependent transcription terminators participate in sophisticated genetic regulatory mechanisms, in both bacteria and phages; they occur in regulatory regions preceding the coding sequences of genes and within coding sequences, as well as at the end of transcriptional units, to prevent readthrough transcription. Most Rho-dependent terminators have been found in enteric bacteria, but they also occur in Gram-positive bacteria and may be widespread among bacteria. Rho-dependent termination requires both cis-acting elements, on the mRNA, and trans-acting factors. The only cis-acting element common to Rho-dependent terminators is richness in rC residues. Additional sequence elements have been observed at different Rho termination sites. These 'auxiliary elements' may assist in the termination process; they differ among terminators, their occurrence possibly depending on the function and sequence context of the terminator. Specific nucleotides required for termination have also been identified at Rho sites. Rho is the main factor required for termination; it is a ring-shaped hexameric protein with ATPase and helicase activities. NusG, NusA and NusB are additional factors participating in the termination process. Rho-dependent termination occurs by binding of Rho to ribosome-free mRNA, C-rich sites being good candidates for binding. Rho's ATPase is activated by Rho-mRNA binding, and provides the energy for Rho translocation along the mRNA; translocation requires sliding of the message into the central hole of the hexamer. When a polymerase pause site is encountered, the actual termination occurs, and the transcript is released by Rho's helicase activity. Many aspects of this process are still being studied. The isolation of mutants suppressing termination, site-directed mutagenesis of cis-acting elements in Rho-dependent termination, and biochemistry, are and will be contributing to unravelling the still undefined aspects of the Rho termination machinery. Analysis of the more sophisticated regulatory mechanisms relying on Rho-dependent termination may be crucial in identifying new essential elements for termination.
Promoters located within the TnlO insertion element cause transcription of"host" sequences adjacent to both ends of the inserted TnlO element. These promoters are usually not observed in genetic experiments because their transcripts are efficiently terminated at nearby p-dependent termination sites. The observations presented here provide an explanation for several confusing aspects of transposon behavior and suggest the possibility that many transposons possess promoters that have escaped detection for similar reasons.Insertion of the transposon TnlO within most bacterial operons has been found to cause an absolute polar block to transcription ofdownstream genes in the operon. This extremely strong polar effect has led to the assumptions that host transcription is terminated and that no transcripts emanate from TnlO into adjacent sequences. In contrast to TnlO's absolutely polar effect on most operons, TnlO insertions in ribosomal RNA operons permit continued transcription of distal sequences (1). These conflicting observations can be reconciled if TnlO provides a promoter whose transcripts are efficiently terminated in some but not all situations. This paper presents evidence that TnlO provides a promoter for transcription ofadjoining sequences. These transcripts have not been detected in the his operon because of the strong natural polarity effects that cause termination of the message. Based on independent results, Blazey and Bums (2) have inferred that the TnlO elements cause transcription of genes adjacent to insertion sites in the ilv operon. Possible implications of these results for other transposons will be discussed.When TnlO insertion mutations were first characterized in operons (3-5) it was noted that most insertions were absolutely polar. In the his operon of Salmonella typhimurium, this conclusion was based on complementation tests. Insertions in the promoter-proximal gene hisG block expression of the immediately adjacent hisD and hisC genes (see Fig.
Previous genetic analysis showed that the polar effects of mutations in the hisG cistron of Salmonella typhimurium are dependent on the presence of a single putative transcription termination element within the hisG gene. In fact, all proximal mutations causing translation termination are strongly polar, whereas distal ones are not. The element was mapped by isolating mutations able to relieve the polar phenotype, and they were found to be small deletions in the region downstream of the translational stop codon (M. S. Ciampi and J. R. Roth, Genetics 118:193-202, 1988). In this study, we analyzed the his-specific RNAs synthesized in vivo in different strains harboring the polar frameshift hisG2148 mutation. The nature of the polarity effects is clearly transcriptional, since shorter RNA molecules were produced. When the hisG2148 mutation was transferred in a rho background or in strains harboring the small distal deletions, an increase in readthrough transcription was observed. The transcriptional termination element was characterized in more detail by performing high-resolution S1 nuclease mapping experiments. This analysis showed that (i) termination or exonucleolytic degradation following termination produced transcripts with heterogeneous 3' ends; (ii) this process is dependent on the transcription termination factor Rho, since relief of termination occurs in a rho background; and (iii) the element appears to function as a transcription terminator, at least to some extent, even in the course of active translation of the hisG cistron.
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