March 21, 2006 ͉ vol. 103 ͉ no. 12 ͉ 4331–4794 In This Issue Proceedings of the National Academy ofPNAS Sciences of the United States of America www.pnas.org 4362 Trapping single biomolecules in free solution 4377 Interconnection of tropospheric ozone and sulfate 4457 Resolution measure and single-molecule microscopy 4570 Non-Watson–Crick mispairs in tRNA promote translation 4634 Alfalfa nodule bacterium illuminates vitamin B12 synthesis CHEMISTRY, BIOPHYSICS GEOPHYSICS Trapping single biomolecules Interconnection of tropospheric ozone in free solution and sulfate Adam Cohen and W. E. Moerner report on the successful sup- Nadine Unger et al. show that ozone (O3) precursor emissions can pression of Brownian motion of individual biomolecules in free dramatically affect both air quality and climate forcing by increas- solution. The authors used an anti-Brownian electrokinetic ing the levels of tropospheric sulfate (SO4). Ozone and sulfate are (ABEL) trap, which monitors strongly coupled through their the Brownian motion of photochemistry; sulfate is gen- a nanoparticle through fluo- erated by the oxidation of rescence microscopy and sulfur dioxide (SO2) by the hy- applies a feedback voltage to droxyl radical (OH) or hydro- a microfluidic cell so that the gen peroxide (H2O2), both of resulting electrophoretic and which can be derived from O3. electroosmotic forces produce Likewise, O3 production re- a drift that cancels the Brown- quires the presence of nitrogen Difference in surface sulfate mixing ian motion of the particle. oxides, which sulfate can re- ratio between future control simu- The ABEL technology was move by conversion to nitric lation and sensitivity simulation used to trap individual parti- acid. Using an atmospheric with future SO2 emissions but Measured trajectories of 13 trapped cles of fluorescently labeled composition–climate model, present-day O3 precursor emissions. particles of TMV. tobacco mosaic virus (TMV), Unger et al. investigated how measuring Ϸ300 nm in length this interaction might affect future climate change. They found and 15 nm in diameter. Cohen and Moerner were also able to that, in 2030, increases in ozone precursor emissions will raise sur- trap individual Ϸ100-nm-diameter vesicles of egg-phosphatidyl- face sulfate levels in several regions, including a 20% increase over choline that had been doped with one to two fluorescent lipid the Indian subcontinent. In the model, ozone precursor emissions molecules to create a low fluorescent intensity. To trap objects contribute 20% of the negative sulfate forcing in this region, which Ͻ20 nm in diameter, a higher-viscosity trapping medium was is more than twice the direct positive forcing of ozone itself. In used. By using a solution of 50% glycerol, single molecules of contrast, changes in sulfate precursor emissions do not significantly large proteins (GroEL and B-phycoerythrin) and single fluores- affect future ozone levels. This interconnection between ozone and cent CdSe nanocrystals were trapped. The ABEL technology sulfate can complicate environmental efforts, because a reduction may enable advanced biophysical measurements, such as provid- in ozone precursors would improve surface air quality but would ing a nonperturbative way to study the folding of single proteins also impose additional positive forcing through sulfate reduction. in solution. — R.N. The authors suggest that future regulations should address ozone and sulfate simultaneously. — N.Z. ‘‘Suppressing Brownian motion of individual biomolecules in solu- tion’’ by Adam E. Cohen and W. E. Moerner (see pages 4362–4365) ‘‘Cross influences of ozone and sulfate precursor emissions changes on air quality and climate’’ by Nadine Unger, Drew T. Shindell, Dorothy M. Koch, and David G. Streets (see pages 4377–4380) www.pnas.org͞cgi͞doi͞10.1073͞iti1206103 PNAS ͉ March 21, 2006 ͉ vol. 103 ͉ no. 12 ͉ 4331–4332 Downloaded by guest on October 2, 2021 six mutant G-U and A-C mispairs in Escherichia coli tRNAGly BIOPHYSICS, ENGINEERING and determined the in vivo aminoacylation specificity and translational property of the mutants. The mispairs boosted Resolution measure and single-molecule aminoacylation and translation primarily because they acti- microscopy vated tRNA through their conformational flexibility. Conver- Sripad Ram et al. developed a resolution measure that over- sion of the G-U mispair to the Watson–Crick G-C pair was comes limitations encountered by applying Rayleigh’s crite- shown to inactivate tRNA function. The author suggests that, Gly rion to single-molecule microscopy, providing a quantitative because the six mispair sites are preserved across tRNA tool for designing and evaluating experiments probing biomo- in many organisms, a basic structure–function signature Gly lecular interactions. It is commonly held that Rayleigh’s crite- exists within tRNA , with analogous functions in other rion imposes a limit on the study of molecular interactions at RNAs. — F.A. Ͻ distances of 200 nm. Yet, this criterion was formulated in ‘‘Surprising contribution to aminoacylation and translation of non- a deterministic setting that neglects the stochastic nature of Watson–Crick pairs in tRNA’’ by William H. McClain (see pages the photon detection process and thus fails to consider the 4570–4575) total number of photons in the acquired data. Ram et al. worked within a stochastic framework and developed an alter- MICROBIOLOGY native resolution measure, which is given in terms of quanti- ties such as the expected number of detected photons, numer- Alfalfa nodule bacterium illuminates ical aperture of the objective lens, and wavelength of the vitamin B12 synthesis detected photons. This measure, unlike Rayleigh’s criterion, predicts that the resolution of a microscope can be improved An unusually fluorescent bacterial colony has yielded insight by increasing the number of photons collected from the point into a previously unresolved step in the synthesis of vitamin sources. The authors presented experimental results in which B12, Gordon Campbell et al. report. Cobalamin, or vitamin B12, distances from images of closely spaced single molecules were is synthesized only by certain estimated. These results indicated that distances well below bacterial species, such as Sino- Rayleigh’s resolution limit could be determined with an accu- rhizobium meliloti, a bacterium racy as specified by the new resolution measure. — R.N. that lives symbiotically within the roots of the alfalfa plant. ‘‘Beyond Rayleigh’s criterion: A resolution measure with application Although most aspects of to single-molecule microscopy’’ by Sripad Ram, E. Sally Ward, and cobalamin synthesis are Raimund J. Ober (see pages 4457–4462) known, the source of cobal- amin’s lower ligand, 5,6-di- GENETICS, STATISTICS methylbenzimidazole (DMB), Alfalfa nodules elicited by is not well understood. In a S. meliloti. Non-Watson–Crick mispairs in tRNA genetic screen of S. meliloti promote translation mutants, Campbell et al. identified a highly fluorescent mutant William McClain reports that with a severe symbiotic defect: it was unable to fix nitrogen non-Watson–Crick mispairs when inoculated into alfalfa plants. The gene responsible for in tRNA actually enhance the mutant phenotype resembled the bluB gene from aminoacylation and transla- Rhodobacter capsulatus, which is involved in cobalamin synthe- tion. Previous research has sis. Treatment with vitamin B12 restored the bacterium’s ability shown that non-Watson– to establish symbiosis with alfalfa. The mutant produced cobal- Crick mispairs, such as G-U amin only when supplemented with DMB, suggesting that BluB and A-C, are infrequent yet is involved in the synthesis of DMB. The S. meliloti bluB gene’s fundamental units of RNA role in DMB biosynthesis thus provides insight into this previ- secondary structure found in ously unsolved aspect of cobalamin synthesis. — M.M. nearly every class of RNA. ‘‘Sinorhizobium meliloti bluB is necessary for production of 5,6- However, the functional im- dimethylbenzimidazole, the lower ligand of B12’’ by Gordon R. O. Superposition of compensating portance of these mispair Campbell, Michiko E. Taga, Kavita Mistry, Javier Lloret, Peter J. mutations in tRNA. sites has not been well under- Anderson, John R. Roth, and Graham C. Walker (see pages stood. McClain characterized 4634–4639) 4332 ͉ www.pnas.org͞cgi͞doi͞10.1073͞iti1206103 Downloaded by guest on October 2, 2021.