Studies on the gene regulation involved in the lytic-lysogenic switch in Staphylococcus aureus temperate bacteriophage Phi11

Antirepressor proteins of bacteriophages are chiefly involved in interfering with the function of the repressor protein and forcing the bacteriophage to adopt the lytic cycle. The genome of Staphylococcus aureus phage, Phi11 has already been sequenced; from the genome sequence, we amplified gp07 gene and analysed its involvement in the developmental pathway of Phi11. Our results indicate that Gp07 functions as a novel antirepressor and regulates the developmental pathway of Phi11 by enhancing the binding of the Cro repressor protein to its cognate operator. We also report our finding that the CI repressor protein of Phi11 binds to the putative operator of Gp07 and regulates its expression. We further report that S.aureus transcriptional repressor LexA and coprotease RecA play a crucial role in the lytic-lysogenic switching in Phi11. We also identified that the N-terminal domain (Bro-N) of Gp07 is actually responsible for enhancing the binding of Cro repressor to its cognate operator. Our results suggest that Phi11 prophage induction is different from other bacteriophages. This study furnishes a first-hand report regarding the regulation involved in the developmental pathway of Phi11.

Cloning, overexpression and purification of a novel two-domain protein of Staphylococcus aureus phage Phi11

The genome of aureophage Phi11 reveals the presence of the gene gp07 which codes for the putative antirepressor protein (GenBank accession no. NC_004615.1). Antirepressor proteins are mainly involved in lytic cycle determination mechanisms of various bacteriophages. The Phi11 protein Gp07 consists of two domains-an amino terminal Bro domain and a carboxy terminal KilA domain. Despite the important role of antirepressor proteins in the developmental pathway of phages, there are no reports on the purification and characterization of aureophage antirepressor proteins. Here we describe a method to clone, overexpress and purify the full length Gp07 as carboxy terminal hexa histidine tag variant. The recombinant protein was expressed in Escherichia coli BL21(λDE3) cells and gradient of imidazole and NaCl were used for successful purification of the soluble recombinant protein to homogeneity. The protein exists as a dimer in solution as is evident from our gel filtration chromatography and glutaraldehyde cross-linking data. Further, we found that temperature has huge impact on the structural conformation of the protein. We expect that the purification of Gp07 will further our work in characterizing the role played by this protein during phage induction.

A novel ejection protein from bacteriophage 80α that promotes lytic growth

Many staphylococcal bacteriophages encode a minor capsid protein between the genes for the portal and scaffolding proteins. In Staphylococcus aureus bacteriophage 80α, this protein, called gp44, is essential for the production of viable phage, but dispensable for the phage-mediated mobilization of S. aureus pathogenicity islands. We show here that gp44 is not required for capsid assembly, DNA packaging or ejection of the DNA, nor for generalized transduction of plasmids. An 80α Δ44 mutant could be complemented in trans by gp44 expressed from a plasmid, indicating that gp44 plays a post-injection role in the host. Our results show that gp44 is an ejection (pilot) protein that is involved in deciding the fate of the phage DNA after injection. Our data are consistent with a model in which gp44 acts as a regulatory protein that promotes progression to the lytic cycle.

The N-terminal domain of the repressor of Staphylococcus aureus phage Φ11 possesses an unusual dimerization ability and DNA binding affinity

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.

Characterization of an unusual cold shock protein from Staphylococcus aureus

Of the three cold shock proteins expressed by Staphylococcus aureus, CspC is induced poorly by cold but strongly by various antibiotics and toxic chemicals. Using a purified CspC, here we demonstrate that it exists as a monomer in solution, possesses primarily β-sheets, and bears substantial structural similarity with other bacterial Csps. Aggregation of CspC was initiated rapidly at temperatures above 40 °C, whereas, the Gibbs free energy of stabilization of CspC at 0 M GdmCl was estimated to be +1.6 kcal mol(-1), indicating a less stable protein. Surprisingly, CspC showed stable binding with ssDNA carrying a stretch of more than three thymine bases and binding with such ssDNA had not only stabilized CspC against proteolytic degradation but also quenched the fluorescence intensity from its exposed Trp residue. Analysis of quenching data indicates that each CspC molecule binds with ∼5 contiguous thymine bases of the above ssDNA and binding is cooperative in nature.

Stabilization of the primary sigma factor of Staphylococcus aureus by core RNA polymerase

The primary sigma factor (sigma(A)) of Staphylococcus aureus, a potential drug target, was little investigated at the structural level. Using an N-terminal histidine-tagged sigma(A) (His-sigma(A)), here we have demonstrated that it exits as a monomer in solution, possesses multiple domains, harbors primarily alpha-helix and efficiently binds to a S. aureus promoter DNA in the presence of core RNA polymerase. While both N- and C-terminal ends of His- sigma(A) are flexible in nature, two Trp residues in its DNA binding region are buried. Upon increasing the incubation temperature from 25 degrees to 40 degrees C, 60% of the input His-sigma(A) was cleaved by thermolysin. Aggregation of His-sigma(A) was also initiated rapidly at 45( degrees )C. From the equilibrium unfolding experiment, the Gibbs free energy of stabilization of His-sigma(A) was estimated to be +0.70 kcal mol(-1). The data together suggest that primary sigma factor of S. aureus is an unstable protein. Core RNA polymerase however stabilized sigma(A) appreciably.