Part:BBa_K2123200

Strong RBS + MerR (regulatory protein) + trp Terminator

Overview:

MerR is an integrate part of Mer Operon a set of genes that grant bacterial resistance to mercury. MerR gene codifies a regulatory protein which ties itself to mer operator (MerO) of the bidirectional promoter region, activating the mer operon expression and therefore the mercury resistance mechanism. In the absence of Hg, MerR is a weak repressor.

Structure and mechanism:

Molecular structure of MerR protein is constituted of 144 individuals amino acids that differ in nine type of residues. Structurally, the regulator presents itself as an homodimer with mass equivalent to 31 kDa. The C and N-terminal extremities are intimately connected in MerR role as regulator. C-terminal is the Hg binding site, mediated by three cysteine residues. Then, N-terminal is linked to the operator (MerO), between -35 and -10 regions, so the recognition site remains inaccessible to RNA polymerase and genes transcription does not begin. MerR is able to attract RNA polymerase to the promoter region, even in absence of Hg(II). The pre-transcriptional complex merR-polymerase remains steady and not producing until Hg(II) is bound to merR’s C site, causing an allosteric change in the protein’s structure. This is propagated to the DNA strain, which unwinds and favors RNA polymerase binding to the promoter, and transcription starts. The formation of pre-transcriptional complex allows quick answer to Hg(II) presence in the medium. The regulatory mechanism of merR and activation of Mer Operon is illustrated below:

Figure 01: Transcription activation of Mer Operon. A) Pre-transcriptional complex, composed by merR, RNA polymerase and the others transcription factors: merR conformity does not allows RNA polymerase recognition, thus Mer Operon remains off. B) Hg binding to terminal C of the regulator promotes conformational changes in the protein and allows RNA polymerase to starts transcription. As soon as Hg(II) is reduced to its volatile form (Hg0), Mer Operon needs to be repressed again. Hg-sulfur bonds settled with repressor is very steady and its unlikely that Hg leaves merR spontaneously. There are two hypothesis on how Mer Operon is repressed when mercury leaves the cell: a merR protein not complexed with Hg(II) bounds to regulation site and discontinues transcription; or the regulatory protein antagonist to merR, merD, takes the role of repressor and inactivates Mer Operon. Sources: BARKAY, T.; MILLER, S. M.; SUMMERS, A. O. Bacterial mercury resistance from atoms to ecosystems. FEMS Microbiology Reviews 27 (2003) 355-384. BROWN, N. L.; STOYANOV J. V.; KIDD, S. P.; HOBMAN, J. L. I. The MerR family of transcriptional regulators. FEMS Microbiology Reviews 27 (2003) 145-163. O’HALLORAN, T. V.; FRANTZ, B.; SHIN, M. K.; RALSTON, D. M.; WRIGHT, J. G. The MerR heavy metal receptor mediates positive activation in a topologically novel transcription complex. Cell 56 (1989) 119-129.

Usage, Methodology and Experiments

Our team (UFAM-UEA_Brazil) worked improving the Mer operon expression to increase bioremediation in E. coli through novel mer promoters sequences. For it, we primarily characterized the MerR expression under control of different promoters from Anderson Collection (BBa_J23100, BBa_J23104, BBa_J23106 e BBa_I142033) through the repression of RFP (BBa_K081014) production, in a synthetic genetic circuit represented bellow.

Sequence and Features

Assembly Compatibility:

• 10
  COMPATIBLE WITH RFC[10]
• 12
  COMPATIBLE WITH RFC[12]
• 21
  COMPATIBLE WITH RFC[21]
• 23
  COMPATIBLE WITH RFC[23]
• 25
  COMPATIBLE WITH RFC[25]
• 1000
  COMPATIBLE WITH RFC[1000]