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FKBP22, a PPIase (peptidyl-prolyl cis–trans isomerase) produced by Escherichia coli, binds FK506 and rapamycin (both immunosuppressive drugs), shares significant homology with the Mip-like virulence factors, and has been thought to carry a long α-helix (namely α3) between its two domains. To understand whether the length of helix α3 plays any role in the structure, function, and stability of FKBP22-like proteins, we studied a recombinant E. coli FKBP22 (rFKBP22) and its four helix α3 mutant variants by various in vitro probes. Of the helix α3 mutants, two were deletion mutants (rFKBP22D5 and rFKBP22D30), whereas the two others were insertion mutants (rFKBP22I3 and rFKBP22I6). Our investigations revealed that the molecular dimensions, dimerization efficiencies, secondary structures, tertiary structures, stabilities, and protein folding abilities of all mutant proteins are different from those of rFKBP22. Conversely, the rapamycin binding affinities of the mutant proteins were affected very little. Urea-induced unfolding of each protein followed a two-state mechanism and was reversible in nature. Interestingly, rFKBP22D30 was the least stable, whereas rFKBP22I3 appeared to be the most stable of the five proteins. The data together suggest that length of helix α3 contributes significantly to the preservation of the structure, function, and stability of E. coli FKBP22. [ABSTRACT FROM AUTHOR]

Assesses the primary cap8 operon promoter region in Staphylococcus aureus. Initiation site of the primary transcript; Potential promoter sequences; Inverted and direct repeats upstream of the promoter; Factors regulating promoter activity.

Describes the molecular characterization and transcriptional analysis of type 8 capsule genes in Staphylococcus aureus. Use of various plasmids containing DNA inserts; Determination of the involvement of open reading frames; Performance of sequencing analysis; Construction of gene-specific chromosomal mutations; Detection of a cap8-specific transcript.

Analyzes the cloning of serotypes of capsular polysaccharide from Staphylococcus aureus. How cloning was done; Materials and techniques used in experiment; Findings of experiment; References.

Examines the nucleotide sequences of two gene clusters involved in the synthesis of Staphylococcus aureus capsular polysaccharides. Definition of staphylococcus aureus; Biochemical structures of the two genes; Chain-length determination genes; Polymerization and export genes.

Cyclophilin (Cyp), a peptidyl-prolyl cis-trans isomerase (PPIase), acts as a virulence factor in many bacteria including Staphylococcus aureus. The enzymatic activity of Cyp is inhibited by cyclosporin A (CsA), an immunosuppressive drug. To precisely determine the unfolding mechanism and the domain structure of Cyp, we have investigated a chimeric S. aureus Cyp (rCyp) using various probes. Our limited proteolysis and the consequent analysis of the proteolytic fragments indicate that rCyp is composed of one domain with a short flexible tail at the C-terminal end. We also show that the urea-induced unfolding of both rCyp and rCyp-CsA is completely reversible and proceeds via the synthesis of at least one stable intermediate. Both the secondary structure and the tertiary structure of each intermediate appears very similar to those of the corresponding native protein. Conversely, the hydrophobic surface areas of the intermediates are comparatively less. Further analyses reveal no loss of CsA binding activity in rCyp intermediate. The thermodynamic stability of rCyp was also significantly increased in the presence of CsA, recommending that this protein could be employed to screen new CsA derivatives in the future. [ABSTRACT FROM AUTHOR]

CapF, a staphylococcal capsule-producing enzyme, binds Zn2+ ion and NADPH using its C-terminal domain (CTD) and N-terminal domain (NTD), respectively. To elucidate the roles of cofactors and domains, we have systematically investigated the related recombinant proteins, rCapF, rCTD, recombinant NTD (rNTD) and the Zn2+-free rCapF/rCTD, Apo-rCapF/Apo-rCTD. The results show that the secondary structure, tertiary structure, shape and surface hydrophobicity of Apo-rCapF and Apo-rCTD are different from those of rCapF and rCTD. The removal of Zn2+ made rCapF thermo-sensitive, whereas both rCTD and Apo-rCTD are thermo-resistant proteins. Further, Apo-rCapF and rCapF existed as the dimers, whereas rCTD and Apo-rCTD formed a mixture of dimers and tetramers in the aqueous solution. Zn2+ maintained the structure of NTD as well. The NADPH binding activity and Cys accessibility of rNTD, rCapF and Apo-rCapF were significantly different from each other. The binding of NADPH to the above three proteins freely occurred, liberated heat at 25°C and increased their diameters. In addition, the structure, stability, shape and oligomerization ability of rNTD, rCTD and rCapF little resembled each other. Collectively, the domains and cofactors of CapF contribute to preserving its conformation, stability, shape and dimerization ability. [ABSTRACT FROM AUTHOR]

Bacteriophage ф11 encodes repressors CI and Cro for executing its growth in Staphylococcus aureus, a human pathogen. There are three homologous operators O1, O2 and O3 between the repressor-expressing genes. While CI binds to O1 and O2, Cro interacts only with O3. To locate additional CI binding operators in ф11, we searched its genome using the O1/O2 sequence as a probe. The results show the presence of a putative CI binding operator (O4) at the 3° end of the cro. O4 differs from O2 and O1 by one base and five bases, respectively. A specific interaction was noticed between O4 and rCI, a recombinant CI. However, O4 shows no interaction with rCro, a chimeric Cro. Additionally, six guanine bases, situated in and around O4, have interacted with rCI. Interestingly, the rCI binding affinity of O4 or O1 is about 15 times higher than that of O2. A comparative study indicates that some bases and structural alteration, unique to O1 and O4, may contribute to their enhanced rCI binding affinity. Collectively, the study has not only broadened the distinct gene regulatory circuit of ф11 but also suggested that it possibly employs a complex mechanism for its development in S. aureus. [ABSTRACT FROM AUTHOR]

The bacterial cell division protein FtsZ has been considered a potential therapeutic target due to its rapid treadmilling that induces cellular wall construction in bacteria. The current study discovered a novel antimicrobial compound, silibinin, a natural flavonolignan and its impact on the recombinant S. aureus FtsZ (SaFtsZ). Silibinin inhibited S. aureus Newman growth in a dose-dependent manner. The IC 50 and MIC values for silibinin were 75 μM and 200 μM, respectively. It had no cytotoxicity against HEK293 cells in vitro. Silibinin also enlarged the bacterial cell morphology by ∼40 folds and showed antibiofilm property. It perturbed the S. aureus membrane potential both at IC 50 conc. and at MIC conc. Further, it inhibited both the polymerization and GTPase activity of SaFtsZ. It did not inhibit tubulin assembly, a eukaryotic FtsZ homolog. A fluorescence quenching study yielded the K d value for SaFtsZ-Silibinin interaction and binding stoichiometry 0.857 ± 0.188 μM and 1:1, respectively. Both in silico study and competition assay indicated that silibinin binds at the GTP binding site on SaFtsZ. The K i value for the silibinin-mediated inhibition of SaFtsZ was 8.8 μM. Therefore, these findings have comprehensively shown the antimicrobial behavior of silibinin on S. aureus Newman cells targeting SaFtsZ. [ABSTRACT FROM AUTHOR]

Bacteriophage Φ11 uses Staphylococcus aureus as its host and, like lambdoid phages, harbors three homologous operators in between its two divergently oriented repressor genes. None of the repressors of Φ11, however, showed binding to all three operators, even at high concentrations. To understand why the DNA binding mechanism of Φ11 repressors does not match that of lambdoid phage repressors, we studied the N-terminal domain of the Φ11 lysogenic repressor, as it harbors a putative helix-turn-helix motif. Our data revealed that the secondary and tertiary structures of the N-terminal domain were different from those of the full-length repressor. Nonetheless, the N-terminal domain was able to dimerize and bind to the operators similar to the intact repressor. In addition, the operator base specificity, binding stoichiometry, and binding mechanism of this domain were nearly identical to those of the whole repressor. The binding affinities of the repressor and its N-terminal domain were reduced to a similar extent when the temperature was increased to 42°C. Both proteins also adequately dislodged a RNA polymerase from a Φ11 DNA fragment carrying two operators and a promoter. Unlike the intact repressor, the binding of the N-terminal domain to two adjacent operator sites was not cooperative in nature. Taken together, we suggest that the dimerization and DNA binding abilities of the N-terminal domain of the Φ11 repressor are distinct from those of the DNA binding domains of other phage repressors. [ABSTRACT FROM AUTHOR]

SaCyp, a cyclophilin having 197 amino acid residues, acts both as a protein-folding catalyst and a virulence factor in Staphylococcus aureus. Interestingly, a region, homologous to the SaCyp region carrying 121–148 amino acid residues, is present in many putative cyclophilins but absent in well-studied cyclophilins. To determine the exact roles of this unusual region in SaCyp and related proteins, we have investigated a deletion mutant (rCypΔ) of a recombinant SaCyp (rCyp) using various probes. The data reveal that rCypΔ has significantly less catalytic activity and possesses altered structure and hydrophobic surface compared to rCyp. Conversely, the deletion substantially increased inhibitor binding affinity and altered the shape of rCyp. However, both proteins were unfolded by a non-two-state mechanism in the presence of urea. Additionally, the stability of rCyp was significantly reduced due to the deletion of the residues 121–148. Our MD simulation study also indicated the considerable alteration in structure, shape, and fluctuations of SaCyp due to the removal of the region carrying 121–148 residues. Hence, the atypical region located in SaCyp might be vital for maintaining its unique structure, function, stability, and shape. [ABSTRACT FROM AUTHOR]

Abstract The lysogenic growth of phage ф11 in Staphylococcus aureus is controlled by a repressor (CI) that harbors an N-terminal domain (NTD), and a C-terminal domain (CTD). Previously, NTD, like CI, showed DNA binding activity and dimerized in the aqueous solution. To precisely understand the folding mechanism, function, and the stability of CI, NTD, and CTD, we have investigated their recombinant forms, rCI, rNTD, and rCTD, using various probes. The data reveal that rCTD, like rCI and rNTD, is a well-structured protein and produces dimers in the aqueous environment. However, the stability order of the dimers appears to be rCI > rCTD > rNTD. Interestingly, the stability of rNTD or rCTD looks slightly higher than that of rCI. The urea-induced equilibrium unfolding of these proteins proceeded via the production of two intermediates. The structure, surface hydrophobicity, and the dimeric status of one intermediate mostly differed from those of another intermediate or the native protein. Our MD simulation study on the representative NTD shows the substantial change in its structure and stability at the urea concentrations, which formed rNTD intermediates. Collectively, the computational data have supported the experimental data and indicated that the CI and its domains are folded by a similar multiphasic pathway. [ABSTRACT FROM AUTHOR]

Functional up-regulation of heat shock factor 1 (HSF1) activity through different posttranslational modifications has been implicated in the survival and proliferation of various cancers. It is increasingly recognized that the HSF1 gene is also up-regulated at the transcriptional level, a phenomenon correlated with poor prognosis for patients with different cancers, including breast cancer. Here, we analyzed the transcriptional up-regulation of HSF1 in human cells upon arsenite- or peroxide-induced oxidative stress. Sequential promoter truncation coupled with bioinformatics analysis revealed that this activation is mediated by two antioxidant response elements (AREs) located between 1707 and 1530 bp upstream of the transcription start site of the HSF1 gene. Using shRNA-mediated down-regulation, ChIP of NRF2, site-directed mutagenesis of the AREs, and DNase I footprinting of the HSF1 promoter, we confirmed that nuclear factor erythroid-derived 2-like 2 (NRF2, also known as NFE2L2) interacts with these AREs and up-regulates HSF1 expression. We also found that BRM/SWI2-related gene 1 (BRG1), a catalytic subunit of SWI2/SNF2-like chromatin remodeler, is involved in this process. We further show that NRF2-dependent HSF1 gene regulation plays a crucial role in cancer cell biology, as interference with NRF2-mediated HSF1 activation compromised survival, migration potential, and the epithelial-to-mesenchymal transition and autophagy in MCF7 breast cancer cells exposed to oxidative stress. Taken together, our findings unravel the mechanistic basis of HSF1 gene regulation in cancer cells and provide molecular evidence supporting a direct interaction between HSF1 and NRF2, critical regulators of two cytoprotective mechanisms exploited by cancer cells. [ABSTRACT FROM AUTHOR]

SarA, a winged-helix DNA binding protein, is a global virulence regulator in Staphylococcus aureus . The putative DNA binding region of SarA is located between amino acid residues Leu 53 and Gln 97. Previous studies have demonstrated that residues at positions 84, 88, 89, and 90 are critical for its function. To precisely understand the roles of the DNA binding residues, we have investigated nine mutants of a recombinant SarA (rSarA) along with the rSarA mutants carrying mutations at the above four positions. Of the thirteen mutants, eleven mutants show weaker DNA binding activity in vitro compared to rSarA. As noted earlier, the DNA binding affinity of rSarA was maximally affected due to the mutation at position 84 or 90. Each of the functionally-defective mutants also possesses an altered structure and stability. Additionally, the mutations at positions 84 and 90 have severely affected the formation of hydrogen (H) bonds at the interface between SarA and the cognate DNA. The mutation at position 64 also has perturbed the generation of some interface H-bonds. Therefore, the disruption of H-bonds in the protein-DNA interface and the structural alteration in the protein may be responsible for the reduced DNA binding activity of the mutants. [ABSTRACT FROM AUTHOR]

RsbW, an anti-sigma factor possessing kinase activity, is expressed by many Gram-positive bacteria including Staphylococcus aureus. To obtain clues about the domain structure and the folding-unfolding mechanism of RsbW, we have elaborately studied rRsbW, a recombinant S. aureus RsbW. Sequence analysis of the protein fragments, generated by the limited proteolysis of rRsbW, has proposed it to be a single-domain protein. The unfolding of rRsbW in the presence of GdnCl or urea was completely reversible in nature and occurred through the formation of at least two intermediates. The structure, shape, and the surface hydrophobicity of no intermediate completely matches with those of other intermediates or the native rRsbW. Interestingly, one of the intermediates, formed in the presence of less GdnCl concentrations, has a molten globule-like structure. Conversely, all of the intermediates, like native rRsbW, exist as dimers in aqueous solution. The putative molten globule and the urea-generated intermediates also have retained some kinase activity. Additionally, the putative ATP binding site/catalytic site of rRsbW shows higher denaturant sensitivity than the tentative dimerization region of this enzyme. [ABSTRACT FROM AUTHOR]

SarA, a pleiotropic transcription regulator, is encoded by Staphylococcus aureus, a pathogenic bacterium. The expression of many virulence and non-virulence genes in S. aureus is modulated by this regulator. Structural studies have shown it to be a winged-helix DNA-binding protein carrying two monomers. Each SarA monomer is composed of five α-helices (α1-α5), three β-strands (β1-β3) and multiple loops. The putative DNA binding region of SarA is constituted with α3, α4, β2, and β3, whereas, its dimerization seems to occur using α1, α2, and α5. Interestingly, many SarA-like proteins are dimeric and use three or more helices for their dimerization. To clearly understand the roles of helix α1 in the dimerization, we have constructed and purified a SarA mutant (Δα1) that lacks helix α1. Our in-depth studies with Δα1 indicate that the helix α1 is critical for preserving the structure, DNA binding activity and thermodynamic stability of SarA. However, the helix has little affected its dimerization ability. Possible reasons for such anomaly have been discussed at length. [ABSTRACT FROM AUTHOR]

The FKBP22 and the related peptidyl-prolyl cis – trans isomerases dimerize using their N-terminal domains. Conversely, their C-terminal domains possess both the substrate and inhibitor binding sites. To delineate the roles of a conserved Tyr residue at their N-terminal domains, we have studied a FKBP22 mutant that carries an Ala in place of the conserved Tyr at position 15. We have demonstrated that the Tyr 15 of FKBP22 is indispensable for preserving its dimerization ability, catalytic activity, and structure. The residue, however, little contributed to its inhibitor binding ability and stability. The mode of action of Tyr 15 has been discussed at length. [ABSTRACT FROM AUTHOR]

Triton X-100 (TX-100), a useful non-ionic surfactant, reduced the methicillin resistance in Staphylococcus aureus significantly. Many S. aureus proteins were expressed in the presence of TX-100. SarA, one of the TX-100-induced proteins, acts as a global virulence regulator in S. aureus. To understand the effects of TX-100 on the structure, and function of SarA, a recombinant S. aureus SarA (rSarA) and its derivative (C9W) have been investigated in the presence of varying concentrations of this surfactant using various probes. Our data have revealed that both rSarA and C9W bind to the cognate DNA with nearly similar affinity in the absence of TX-100. Interestingly, their DNA binding activities have been significantly increased in the presence of pre-micellar concentration of TX-100. The increase of TX-100 concentrations to micellar or post-micellar concentration did not greatly enhance their activities further. TX-100 molecules have altered the secondary and tertiary structures of both proteins to some extents. Size of the rSarA-TX-100 complex appears to be intermediate to those of rSarA and TX-100. Additional analyses show a relatively moderate interaction between C9W and TX-100. Binding of TX-100 to C9W has, however, occurred by a cooperative pathway particularly at micellar and higher concentrations of this surfactant. Taken together, TX-100-induced structural alteration of rSarA and C9W might be responsible for their increased DNA binding activity. As TX-100 has stabilized the somewhat weaker SarA-DNA complex effectively, it could be used to study its structure in the future. [ABSTRACT FROM AUTHOR]

SarA, a Staphylococcus aureus-specific dimeric protein, modulates the expression of numerous proteins including various virulence factors. Interestingly, S. aureus synthesizes multiple SarA paralogs seemingly for optimizing the expression of its virulence factors. To understand the domain structure/flexibility and the folding/unfolding mechanism of the SarA protein family, we have studied a recombinant SarA (designated rSarA) using various in vitro probes. Limited proteolysis of rSarA and the subsequent analysis of the resulting protein fragments suggested it to be a single-domain protein with a long, flexible C-terminal end. rSarA was unfolded by different mechanisms in the presence of different chemical and physical denaturants. While urea-induced unfolding of rSarA occurred successively via the formation of a dimeric and a monomeric intermediate, GdnCl-induced unfolding of this protein proceeded through the production of two dimeric intermediates. The surface hydrophobicity and the structures of the intermediates were not identical and also differed significantly from those of native rSarA. Of the intermediates, the GdnCl-generated intermediates not only possessed a molten globule-like structure but also exhibited resistance to dissociation during their unfolding. Compared to the native rSarA, the intermediate that was originated at lower GdnCl concentration carried a compact shape, whereas, other intermediates owned a swelled shape. The chemical-induced unfolding, unlike thermal unfolding of rSarA, was completely reversible in nature. [ABSTRACT FROM AUTHOR]

FKBP22, an Escherichia coli-encoded PPIase (peptidyl-prolyl cis-trans isomerase) enzyme, shares substantial identity with the Mip-like pathogenic factors, caries two domains, exists as a dimer in solution and binds some immunosuppressive drugs (such as FK506 and rapamycin) using its C-terminal domain (CTD). To understand the effects of these drugs on the structure and stability of the Mip-like proteins, rFKBP22 (a chimeric FKBP22) and CTD+ (a CTD variant) have been studied in the presence and absence of rapamycin using different probes. We demonstrated that rapamycin binding causes minor structural alterations of rFKBP22 and CTD+. Both the proteins (equilibrated with rapamycin) were unfolded via the formation of intermediates in the presence of urea. Further study revealed that thermal unfolding of both rFKBP22 and rapamycin-saturated rFKBP22 occurred by a three-state mechanism with the synthesis of intermediates. Intermediate from the rapamycin-equilibrated rFKBP22 was formed at a comparatively higher temperature. All intermediates carried substantial extents of secondary and tertiary structures. Intermediate resulted from the thermal unfolding of rFKBP22 existed as the dimers in solution, carried an increased extent of hydrophobic surface and possessed relatively higher rapamycin binding activity. Despite the formation of intermediates, both the thermal and urea-induced unfolding reactions were reversible in nature. Unfolding studies also indicated the considerable stabilization of both proteins by rapamycin binding. The data suggest that rFKBP22 or CTD+ could be exploited to screen the rapamycin-like inhibitors in the future. [ABSTRACT FROM AUTHOR]

ClpC is an ATPase chaperone found in most Gram-positive Iow-GC bacteria. It has been recently reported that ClpC affected virulence gene expression in Staphylococcus aureus. Here we report that ClpC regulates transcription of the cap operon and accumulation of capsule, a major virulence factor for S. aureus. As virulence genes are regulated by a complex regulatory network in S. aureus, we have used capsule as a model to understand this regulation. By microarray analyses of strain Newman, we found that ClpC strongly activates transcription of the sae operon, whose products are known to negatively regulate capsule synthesis in this strain. Further studies indicated that ClpC repressed capsule production by activating the sae operon in strain Newman. Interestingly, the clpC gene cloned into a multiple-copy plasmid vector exhibited an activation phenotype, suggesting that ClpC overexpression has a net positive effect. In the absence of sae function, by either deletion or correction of a native mutation within saeS, we found that ClpC had a positive effect on capsule production. Indeed, in the UAMS-1 strain, which does not have the saeS mutation, ClpC functioned as an activator of capsule production. Our microarray analyses of strain Newman also revealed that CodY, a repressor of capsule production, was repressed by ClpC. Using genetic approaches, we showed that CodY functioned downstream of ClpC, leading to capsule activation both in Newman and in UAMS-1. Thus, ClpC functions in two opposite pathways in capsule regulation in strain Newman but functions as a positive activator in strain UAMS-1. [ABSTRACT FROM AUTHOR]

Previously, the repressor protein of mycobacteriophage L1 bound to two operator DNAs with dissimilar affinity. Surprisingly, the putative operator consensus sequence, 5′GGTGGa/cTGTCAAG, lacks the dyad symmetry reported for the repressor binding operators of λ and related phages. To gain insight into the structure of the L1 repressor-asymmetric operator DNA complex, we have performed various in vitro experiments. A dimethyl sulfate protection assay revealed that five guanine bases, mostly distributed in the two adjacent major grooves of the 13 bp operator DNA helix, participate in repressor binding. Hydroxyl radical footprinting demonstrated that interaction between the repressor and operator DNA is asymmetric in nature and occurs primarily through one face of the DNA helix. Genetic studies not only confirmed the results of the dimethyl sulfate protection assay but also indicated that other bases in the 13 bp operator DNA are critical for repressor binding. Interestingly, repressor that weakly induced bending in the asymmetric operator DNA interacted with this operator as a monomer. The tertiary structure of the L1 repressor-operator DNA complex therefore appears to be distinct from those of the lambdoid phages even though the number of repressor molecules per operator site closely matched that of the λ phage system. [ABSTRACT FROM AUTHOR]

The repressor protein and cognate operator DNA of any temperate Staphylococcus aureus phage have not been investigated in depth, despite having the potential to enrich the molecular biology of the staphylococcal system. In the present study, using the extremely pure repressor of temperate Staphylococcus aureus phage φ11 (CI), we demonstrate that CI is composed of α-helix and β-sheet to a substantial extent at room temperature, possesses two domains, unfolds at temperatures above 39 °C and binds to two sites in the φ11 cI- cro intergenic region with variable affinity. The above CI binding sites harbor two homologous 15 bp inverted repeats ( O1 and O2), which are spaced 18 bp apart. Several guanine bases located in and around O1 and O2 demonstrate interaction with CI, indicating that these 15 bp sites are used as operators for repressor binding. CI interacted with O1 and O2 in a cooperative manner and was found to bind to operator DNA as a homodimer. Interestingly, CI did not show appreciable binding to another homologous 15 bp site ( O3) that was located in the same primary immunity region as O1 and O2. Taken together, these results suggest that φ11 CI and the φ11 CI–operator complex resemble significantly those of the lambdoid phages at the structural level. The mode of action of φ11 CI, however, may be distinct from that of the repressor proteins of λ and related phages. [ABSTRACT FROM AUTHOR]

Background: Lysogenic mode of life cycle of a temperate bacteriophage is generally maintained by a protein called 'repressor'. Repressor proteins of temperate lambdoid phages bind to a few symmetric operator DNAs in order to regulate their gene expression. In contrast, repressor molecules of temperate mycobacteriophages and some other phages bind to multiple asymmetric operator DNAs. Very little is known at present about the structure-function relationship of any mycobacteriophage repressor. Results: Using highly purified repressor (CI) of temperate mycobacteriophage L1, we have demonstrated here that L1 CI harbors an N-terminal domain (NTD) and a C-terminal domain (CTD) which are separated by a small hinge region. Interestingly, CTD is more compact than NTD at 25°C. Both CTD and CI contain significant amount of α-helix at 30°C but unfold partly at 42°C. At nearly 200 nM concentration, both proteins form appreciable amount of dimers in solution. Additional studies reveal that CI binds to O64 and OL types of asymmetric operators of L1 with variable affinity at 25°C. Interestingly, repressor - operator interaction is affected drastically at 42°C. The conformational change of CI is most possibly responsible for its reduced operator binding affinity at 42°C. Conclusion: Repressors encoded by mycobacteriophages differ significantly from the repressor proteins of λ and related phages at functional level but at structural level they are nearly similar. [ABSTRACT FROM AUTHOR]