Article

TraA, TraC and TraD autorepress two divergent quorum-regulated promoters near the transfer origin of the Ti plasmid of Agrobacterium tumefaciens.

Department of Microbiology, Cornell University, Ithaca, NY 14850, USA.
Molecular Microbiology (impact factor: 5.01). 04/2007; 63(6):1769-82. DOI:10.1111/j.1365-2958.2007.05624.x pp.1769-82
Source: PubMed

ABSTRACT Whole-genome transcriptional profiling experiments were performed to identify the complete set of TraR-regulated genes in isogenic A. tumefaciens strains containing an octopine-type or nopaline-type Ti plasmid. Most of the known TraR-regulated genes as well as a number of new inducible genes were identified. Surprisingly, some known members of this regulon showed both weaker induction and weak levels of expression than we had predicted based upon earlier studies. In particular, traA was expressed at surprisingly weak levels. Genetic analysis showed that the traAFBH operon is repressed by formation of a putative relaxosome at oriT consisting the TraA, TraC and TraD. These proteins also repressed the divergent traCDGyci operon. TraA was essential for oriT processing, and both TraC and TraD were necessary for the efficient processing, although some processing occurred in their absence. Likewise, Ti plasmid conjugation required TraA, TraF and TraG, and occurred at reduced levels in the absence of TraC or TraD. TraA preferentially acted in cis in repressing the traA and traC promoters and in the processing of oriT, which explains the very high activity of plasmid-borne traA-lacZ fusions reported in previous studies.

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    Article: A dual-signal regulatory circuit activates transcription of a set of divergent operons in Salmonella typhimurium.
    Guang Zhao, Natasha Weatherspoon, Wei Kong, Roy Curtiss, Yixin Shi
    [show abstract] [hide abstract]
    ABSTRACT: We present a molecular mechanism for signal transduction that activates transcription of the SlyA regulon in Salmonella typhimurium. We demonstrate that SlyA mediates transcriptional activation in response to guanosine tetraphosphate, ppGpp, according to the following observations: (i) in vivo transcription of SlyA-dependent genes is repressed when ppGpp is absent; this transcription can be restored by overproducing SlyA; (ii) in vivo dimerization and binding of SlyA to the target promoter are facilitated in the presence of ppGpp; and (iii) in vitro SlyA binding to the target promoter is enhanced when ppGpp is supplemented. Thus, ppGpp must be the cytoplasmic component that stimulates SlyA regulatory function by interacting directly with this regulator in Salmonella. This signaling domain, integrated by the PhoP/PhoQ 2-component system that activates slyA transcription by sensing Mg(2+), forms feedforward loops that regulate chromosomal loci identified through a motif search over the S. typhimurium genome. Many such loci are divergent operons, each formed by 2 neighboring genes in which transcription of these 2 loci proceeds in opposite directions. Both genes, however, are controlled by PhoP and SlyA through a single shared PhoP box and SlyA box present in their intergenic regions. A substitution in either box sequence causes a simultaneous cessation of transcription of a divergent operon, pagD-pagC, equivalent to the phenotype in a phoP or slyA mutant. We also identified several chromosomal loci that possess pagC-type genes without the cognate pagD-type genes. Therefore, our results provide a molecular basis for the understanding of SlyA-dependent phenotypes associated with Salmonella virulence.
    Proceedings of the National Academy of Sciences 01/2009; 105(52):20924-9. · 9.68 Impact Factor
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Keywords

complete
 
divergent traCDGyci operon
 
Genetic analysis
 
isogenic A. tumefaciens strains
 
known members
 
known TraR-regulated genes
 
new inducible genes
 
nopaline-type Ti plasmid
 
oriT
 
oriT processing
 
plasmid-borne traA-lacZ fusions
 
traA
 
TraA preferentially
 
traAFBH operon
 
TraC
 
traC promoters
 
TraR-regulated genes
 
weak levels
 
weaker induction
 
Whole-genome transcriptional profiling experiments