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The single-stranded DNA-binding protein (SSB) of Bacillus subtilis phage phi29 is absolutely required for viral DNA replication in vivo. About approximately 95% of the intrinsic tyrosine fluorescence of phi29 SSB is quenched upon binding to ssDNA, making tyrosine residues strong candidates to be directly involved in complex formation with ssDNA. Thus, we have studied the spectroscopic properties of the phi29 SSB tyrosines (Tyr-50, Tyr-57, and Tyr-76) using steady-state and time-resolved fluorescence measurements. phi29 SSB tyrosines do not seem to be highly restricted by strong interactions with neighbor residues, as suggested by (i) the high value of the average quantum yield of the phi29 SSB fluorescence emission (phiF = 0.067 +/- 0.010), (ii) the fast motions of the tyrosine side chains (phi(short) = 0.14 +/- 0.06 ns), and (iii) the lack of tyrosinate emission at neutral pH. Stern-Volmer analysis of the quenching by acrylamide and I- indicates that phi29 SSB tyrosines are surrounded by a negatively charged environment and located in a relatively exposed protein domain, accessible to the solvent and, likely, to ssDNA. Changes in the intrinsic fluorescence upon ssDNA binding allowed us to determine that temperature has an opposite effect on the thermodynamic parameters K (intrinsic binding constant) and omega (cooperativity) defining phi29 SSB-poly(dT) interaction, the effective DNA binding constant, K(eff) = K omega, being largely independent of temperature. Altogether, the fluorescent properties of phi29 SSB tyrosines are consistent with a direct participation in complex formation with ssDNA.

The strand-displacement mechanism of Bacillus subtilis phage phi29 DNA replication occurs through replicative intermediates with high amounts of single-stranded DNA (ssDNA). These ssDNA must be covered by the viral ssDNA-binding protein, phi29 SSB, to be replicated in vivo. To understand the characteristics of phi29 SSB-ssDNA complex that could explain the requirement of phi29 SSB, we have (i) determined the hydrodynamic behavior of phi29 SSB in solution and (ii) monitored the effect of complex formation on phi29 SSB and ssDNA secondary structure. Based on its translational frictional coefficient (3.5 +/- 0.1) x 10(8) gs(-1), and its rotational correlation time, 7.0 +/- 0.5 ns, phi29 SSB was modeled as a nearly spherical ellipsoid of revolution. The axial ratio (p = a/b) could range from 0.8 to 1.0 (oblate model, a < b) or 1.0 to 3.2 (prolate model, a > b). Far-UV CD spectra, indicated that phi29 SSB is highly organized within a wide range of temperatures (15 to 50 degrees C), being mainly constituted by beta-sheet elements (approximately 50%, at pH 7). Complex formation with ssDNA, although inducing minimal changes on the global conformation of phi29 SSB, had a clear stabilizing effect against pH and temperature increase of the solution samples. On the other hand, phi29 SSB binding leads to non-conservative changes of the near-UV CD spectra of ssDNA, which are consistent with different nearest-neighbor interactions of the nucleotide bases upon complex formation. The above results will be compared to those reported for other SSBs and discussed in terms of the functional roles of phi29 SSB.