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The torsion elastic constant alpha of linear phi 29 DNA has been determined as a function of temperature from 10 to 78 degrees C by studying the decay of the fluorescence polarization anisotropy (FPA) of intercalated ethidium dye. The time-dependent FPA was measured by using a picosecond dye laser for excitation and time-correlated single photon counting detection. Over the region 10-74 degrees C, alpha was effectively constant within experimental error, varying from (3.5 +/- 0.4) X 10(-12) dyn cm at 10 degrees C to (3.7 +/- 0.3) X 10(-12) dyn cm at 74 degrees C. At 78 degrees C, which is just above the melting temperature Tm = 76 degrees C, alpha decreased to (3.3 +/- 0.3) X 10(-12) dyn cm, and at 90 degrees C, where the DNA is completely denatured, both the fluorescence lifetime and the decay time of the FPA are characteristic of unbound ethidium bromide. The weak temperature dependence of alpha implies that DNA torsional deformations do not occur primarily at sites of high enthalpy perturbed structures such as open base pairs.

We have investigated the influence of ionic strength and nucleic acid concentration on the rotational Brownian motion of Escherichia coli tRNA1Val by studying the decay of the fluorescence polarization anisotropy (FPA) of intercalated ethidium on a nanosecond time scale. The rotational relaxation time tau R remains essentially constant as the ionic strength is varied from 2 to 100 mM at a tRNA concentration of 54 mg/mL. tau R also remains practically unchanged as the tRNA concentration is varied from 0.3 to 54 mg/mL at an ionic strength of 130 mM. Present hydrodynamic theories generally predict a more pronounced concentration dependence for rotational diffusion than we observe. This disagreement may result from a nonrandom distribution of the tRNA molecules in solution due to electrostatic interactions. By combining independent data from time-resolved nuclear Overhauser effect (NOE) cross-relaxation experiments and FPA experiments on the same tRNA, we are able to estimate the interproton spacing for the guanine N1-H and the uracil N3-H of the GU-50 base pair in E. coli tRNA1Val. This distance is 0.272 nm.

Salmonella enterica serovar Heidelberg (SH) is one of the prolific serovars causing poultry-associated food-borne illness in the world. Their ability to cause invasive infections and their promiscuity to plasmids that confer multidrug resistance to antibiotics of human health importance makes them a public health threat. Although, horizontal gene transfer (HGT) is recognized as the major mechanism used by Salmonella for acquiring antimicrobial resistance (AR) and virulence genes, the biology behind acquisition of new genes in SH is still unknown. In this study, we show that one day old broiler chicks challenged orally or via the cloaca with an antibiotic susceptible SH strain and raised without antibiotics carried susceptible and multidrug resistance SH strains 14 days after challenge. SH infection perturbed the bacterial community of broiler chicks and orally challenged chicks acquired AR at a higher rate than chicks challenged through the cloaca. Furthermore, SH strains lost and gained new genes, while some inverted their chromosome after colonizing the gut of broiler chicks. The acquisition of IncI1 plasmid multilocus sequence type 26 (pST26) from commensal Escherichia coli population present in the gut of broiler chicks conferred multidrug resistance phenotype to SH recipients and carriage of pST26 increased the fitness of SH under acidic selection pressure. Our results suggest that HGT shapes the evolution of AR in SH and that antibiotic use reduction alone is insufficient to limit AR plasmid transfer from commensal bacteria to Salmonella.