Features of the rho-dependent transcription termination polar element within the hisG cistron of Salmonella typhimurium

Abstract

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.

Rho-dependent terminators and transcription termination

“…Similar 'auxiliary elements' have been identified in the his operon of Salmonella (Ciampi et al, 1989;Carlomagno & Nappo, 2001) and in the tna operon of E. coli (Gong & Yanofsky, 2002) (Table 1), where they seem to be required for Rho-dependent termination.…”

“…In vivo analysis of the transcripts produced by the polar mutant unveiled the existence of five major transcription end points in hisG, all affected by Rho to a different extent (Ciampi et al, 1989). Three of these sites fall immediately downstream of sequences with the potential for forming secondary structures (Ciampi et al, 1989) and one of them, the first in the gene and the most affected in a rho background, occurs within a sequence (Ciampi et al, 1989) resembling the boxA consensus element.…”

“…Three of these sites fall immediately downstream of sequences with the potential for forming secondary structures (Ciampi et al, 1989) and one of them, the first in the gene and the most affected in a rho background, occurs within a sequence (Ciampi et al, 1989) resembling the boxA consensus element. This Salmonella his sequence, cloned on a plasmid, was shown to enhance termination by interacting with the NusB factor of E. coli cells (Carlomagno & Nappo, 2001).…”

“…Similar rut elements have been found at the trp t9 terminator of E. coli (Zalatan et al, 1993). Some Rho-dependent terminator regions show sequences with homology to boxA (Friedman & Olson, 1983), which has an antitermination function in the bacteriophage l (Das, 1992) and in other systems (Heinrich et al, 1995;Vogel & Jensen, 1995); interestingly, boxA-like sequences in the tna operon of E. coli (Gong & Yanofsky, 2002) and in the his operon of Salmonella typhimurium (Ciampi et al, 1989;Carlomagno & Nappo, 2001) seem to be required for Rho-dependent termination. These and other potential 'auxiliary elements' (listed in Table 1), which are not common features of Rho-dependent terminators, could be part of a pool of elements, different combinations of which may contribute to Rho loading efficiency and to the efficiency of termination.…”

The nature of the traK4 mutation in the F sex factor of Escherichia coli

“…Unlike rho-independent terminators, which usually occur between genes, rho-dependent terminators have been identified both between genes, as in AtR, (31) and trpt' (18), and within an operon, as in the his operon of Salmonella typhimurium (4,6) and the ilvGMEDA operon (32) and lacZ gene (22) of E. coli. In the latter two cases, the polarity of certain mutations has been attributed to rho-dependent termination under conditions in which translation has stopped.…”