A DNA sequence for the genome of bacteriophage phi X174 of approximately 5,375 nucleotides has been determined using the rapid and simple 'plus and minus' method. The sequence identifies many of the features responsible for the production of the proteins of the nine known genes of the organism, including initiation and termination sites for the proteins and RNAs. Two pairs of genes are coded by the same region of DNA using different reading frames.

The MerR family is a group of transcriptional activators with similar N-terminal helix-turn-helix DNA binding regions and C-terminal effector binding regions that are specific to the effector recognised. The signature of the family is amino acid similarity in the first 100 amino acids, including a helix-turn-helix motif followed by a coiled-coil region. With increasing recognition of members of this class over the last decade, particularly with the advent of rapid bacterial genome sequencing, MerR-like regulators have been found in a wide range of bacterial genera, but not yet in archaea or eukaryotes. The few MerR-like regulators that have been studied experimentally have been shown to activate suboptimal sigma(70)-dependent promoters, in which the spacing between the -35 and -10 elements recognised by the sigma factor is greater than the optimal 17+/-1 bp. Activation of transcription is through protein-dependent DNA distortion. The majority of regulators in the family respond to environmental stimuli, such as oxidative stress, heavy metals or antibiotics. A subgroup of the family activates transcription in response to metal ions. This subgroup shows sequence similarity in the C-terminal effector binding region as well as in the N-terminal region, but it is not yet clear how metal discrimination occurs. This subgroup of MerR family regulators includes MerR itself and may have evolved to generate a variety of specific metal-responsive regulators by fine-tuning the sites of metal recognition.

We have shown that the open reading frame ybbI in the genomic sequence of Escherichia coli K‐12 encodes the regulator of expression of the copper‐exporting ATPase, CopA. In vivo studies showed that ybbI (designated cueR for copper export regulator gene) was required for copper tolerance during growth, that disruption of cueR caused loss of copA expression and that copA gene expression was regulated by cueR and by copper or silver ions. Expression of a lacZ reporter gene under the control of the copA promoter was approximately proportional to the concentration of cupric ions in the medium, but increased more rapidly in response to silver ion concentrations. The start of the copA transcript was located by primer extension mapping, and DNase I protection assays showed that the CueR protein binds in vitro to a dyad symmetrical sequence within a 19 bp spacer sequence in the copA promoter. CueR binding occurs in vitro in both the presence and the absence of RNA polymerase with or without copper ions present but, in the presence of CueR, RNA polymerase and copper ions, permanganate‐sensitive transcription complexes were formed. CueR is predicted to have an N‐terminal helix–turn–helix sequence and shows similarity to MerR family regulators.

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