We have investigated protein‐rRNA cross‐links formed in 30S and 50S ribosomal subunits of Escherichia coli and Bacillus stearothermophilus at the molecular level using UV and 2‐iminothiolane as cross‐linking agents. We identified amino acids cross‐linked to rRNA for 13 ribosomal proteins from these organisms, namely derived from S3, S4, S7, S14, S17, L2, L4, L6, L14, L27, L28, L29 and L36. Several other peptide stretches cross‐linked to rRNA have been sequenced in which no direct cross‐linked amino acid could be detected. The cross‐linked amino acids are positioned within loop domains carrying RNA binding features such as conserved basic and aromatic residues. One of the cross‐linked peptides in ribosomal protein S3 shows a common primary sequence motif–the KH motif–directly involved in interaction with rRNA, and the cross‐linked amino acid in ribosomal protein L36 lies within the zinc finger‐like motif of this protein. The cross‐linked amino acids in ribosomal proteins S17 and L6 prove the proposed RNA interacting site derived from three‐dimensional models. A comparison of our structural data with mutations in ribosomal proteins that lead to antibiotic resistance, and with those from protein‐antibiotic cross‐linking experiments, reveals functional implications for ribosomal proteins that interact with rRNA.

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Abstract. Aerodynamic particle size spectrometers are a well-established method to measure number size distributions of coarse mode particles in the atmosphere. Quality assurance is essential for atmospheric observational aerosol networks to obtain comparable results with known uncertainties. In a laboratory study within the framework of ACTRIS (Aerosols, Clouds, and Trace gases Research Infrastructure Network), 15 aerodynamic particle size spectrometers (APS model 3321, TSI Inc., St. Paul, MN, USA) were compared with a focus on flow rates, particle sizing, and the unit-tounit variability of the particle number size distribution.Flow rate deviations were relatively small (within a few percent), while the sizing accuracy was found to be within 10 % compared to polystyrene latex (PSL) reference particles. The unit-to-unit variability in terms of the particle number size distribution during this study was within 10 % to 20 % for particles in the range of 0.9 up to 3 µm, which is acceptable for atmospheric measurements. For particles smaller than that, the variability increased up to 60 %, probably caused by differences in the counting efficiencies of individual units. Number size distribution data for particles smaller than 0.9 µm in aerodynamic diameter should only be used with caution. For particles larger than 3 µm, the unit-tounit variability increased as well. A possible reason is an insufficient sizing accuracy in combination with a steeply sloping particle number size distribution and the increasing uncertainty due to decreasing counting. Particularly this uncertainty of the particle number size distribution must be considered if higher moments of the size distribution such as the particle volume or mass are calculated, which require the conversion of the aerodynamic diameter measured to a volume equivalent diameter.In order to perform a quantitative quality assurance, a traceable reference method for the particle number concentration in the size range 0.5-3 µm is needed.

The TSI DustTrak Aerosol Monitor is a portable real-time instrument widely used for particulate matter (PM) mass concentrations monitoring. The aim of this work is to report on issues that have arisen from the use of the latest generation models DustTrak DRX (8533 and 8534) in the BREATHE, UPTECH and IMPROVE projects that can compromise data quality. The main issue we encountered was the occurrence of sudden artefact jumps in PM concentration, which can involve an increase from a few to some hundreds of μg·m. These artefact jumps can sometimes be easily recognised ("obvious jump"), while others can be difficult to identify because the difference in the concentrations before and after the jump might be just few μg·m ("possible jump") or because the jump is sustained over the whole monitoring period and only detectable if PM concentrations are simultaneously measured by other instruments ("hidden jump"). Moreover, in areas of relatively low PM levels, the unit reported concentration of 0μg·m for ambient PM concentration or even negative concentration values which may seriously compromise the dataset. These data suggest issues with the detection of low PM concentrations, which could be due to an incorrect instrument offset or the factory calibration setting being inadequate for these PM concentrations. The upward and downward artefact jumps were not related to especially dusty or clean conditions, since they have been observed in many kinds of environments: indoor and outdoor school environments, subway stations and in ambient urban background air. Therefore, PM concentration data obtained with the TSI DustTrak DRX models should be handled with care and meticulously revised before being considered valid. To prevent these issues the use of auto zero module is recommended, so the DustTrak monitor is automatic re-zeroed without requiring the presence of any user.

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An increased number of studies are focusing on the detection and investigation of nanometer-sized particles. One important tool is the Scanning Mobility Particle Sizer (SMPS). In the case of dynamic processes like nucleation events, the lower detection threshold as well as the measurement time are important system parameters. The limiting factors which determine the accuracy of the SMPS are the high voltage accuracy under dynamic conditions and knowledge of the response function of the detector. Especially the quality of the sizing depends on the accuracy of applied voltages in the differential mobility analyzer. High accuracy of the applied voltage as a function of time during fast scanning is required. Short scan times additionally yield a broadening and smearing of the measured size distribution. Here, the performance of the Nano-SMPS using the new TSI classifier (TSI 3082) in combination with either the nano water condensation particle counter (TSI 3788) or the ultrafine condensation particle counter (TSI 3776) was investigated. The focus of this work is the performance of the system at the lower detection limit as a function of the scan time. Fast scan times as short as three seconds were tested and compared to quasi-stationary measurements. The reproducibility of the aerosol size distribution and number concentration for different distributions and materials was investigated. The tested substances included different proteins (myogobin, ubiquitin, ferritin, albumin), sucrose and polystyrene latex reference particles. The sizing reproducibility and accuracy for all tested scan times was within 3%. This is only possible, by determining the delay time precisely. In addition, the limitations in size resolving power due to smearing and broadening resulting from fast scan times were investigated and quantified. The measured total number concentration was captured with a precision of ±3% for all tested scan times. The determination of the absolute concentration for this size range was not considered and remains a challenge for future studies, as diffusion losses, charging probability and CPC counting efficiency are critical issues.

In an attempt to gain information about the peptidyl transferase center at the peptide level we cross-linked the spiramycin derivative dihydrospiramycin to its functional binding site in the 70 S ribosome of Escherichia coli. In this manner ribosomal proteins S12, S14, L17, L18, L27 and L35 were found specifically affinitylabeled.