Single-chain recombinant human cytomegalovirus protease. Activity against its natural protein substrate and fluorogenic peptide substrates

We report here the production of active recombinant single-chain human cytomegalovirus protease in Escherichia coli and development of a continuous assay for this protease. In order to produce the human cytomegalovirus (HCMV) protease for structural studies and accurate kinetic analysis, mutation of alanine 143 at an internal cleavage site was introduced to prevent auto-proteolysis. The resulting soluble 29-kDa A143Q protease was purified to homogeneity as a stable single-chain protein by hydrophobic interaction and ionic-exchange chromatography. The in vivo protein substrate, assembly protein precursor, was also expressed and purified for activity studies. To develop a continuous protease assay, fluorescent synthetic peptide substrates similar to the cleavage sequence P5 to P5' of the maturation site containing anthranilic acid and nitrotyrosine as a resonance energy transfer donor-acceptor pair were designed. Purified HCMV A143Q protease cleaved the recombinant assembly protein precursor with Km and kcat values of 3.0 +/- 1.0 microM and 13.3 +/- 1.6 min-1. The Km for peptide substrates is at least 45-fold higher than for the natural protein substrate, but the kcat values are similar. A sensitive assay was developed using fluorescent peptide substrates, which can detect nM HCMV protease activity.

Interaction of the Bacillus subtilis glnRA repressor with operator and promoter sequences in vivo

In vivo dimethyl sulfate footprinting of the Bacillus subtilis glnRA regulatory region under repressing and derepressing conditions demonstrated that the GlnR protein, encoded by glnR, interacts with two sites situated within and adjacent to the glnRA promoter. One site, glnRAo1, between positions -40 and -60 relative to the start point of transcription, is a 21-bp symmetrical element that has been identified as essential for glnRA regulation (H. J. Schreier, C. A. Rostkowski, J. F. Nomellini, and K. D. Hirschi, J. Mol. Biol. 220:241-253, 1991). The second site, glnRAo2, is a quasisymmetrical element having partial homology to glnRAo1 and is located within the promoter between positions -17 and -37. The symmetry and extent of modifications observed for each site during repression and derepression indicated that GlnR interacts with the glnRA regulatory region by binding to both sites in approximately the same manner. Experiments using potassium permanganate to probe open complex formation by RNA polymerase demonstrated that transcriptional initiation is inhibited by GlnR. Furthermore, distortion of the DNA helix within glnRAo2 occurred upon GlnR binding. While glutamine synthetase, encoded by glnA, has been implicated in controlling glnRA expression, analyses with dimethyl sulfate and potassium permanganate ruled out a role for glutamine synthetase in directly influencing transcription by binding to operator and promoter regions. Our results suggested that inhibition of transcription from the glnRA promoter involves GlnR occupancy at both glnRAo1 and glnRAo2. In addition, modification of bases within the glnRAo2 operator indicated that control of glnRA expression under nitrogen-limiting (derepressing) conditions included the involvement of a factor(s) other than GlnR.