ALBERT

Description: DNA topology plays a crucial role in all living cells. In prokaryotes, negative supercoiling is required to initiate replication and either negative or positive supercoiling assists decatenation. The role of DNA knots, however, remains a mystery. Knots are very harmful for cells if not removed efficiently, but DNA molecules become knotted in vivo . If knots are deleterious, why then does DNA become knotted? Here, we used classical genetics, high-resolution 2D agarose gel electrophoresis and atomic force microscopy to show that topoisomerase IV (Topo IV), one of the two type-II DNA topoisomerases in bacteria, is responsible for the knotting and unknotting of sister duplexes during DNA replication. We propose that when progression of the replication forks is impaired, sister duplexes become loosely intertwined. Under these conditions, Topo IV inadvertently makes the strand passages that lead to the formation of knots and removes them later on to allow their correct segregation.

Description: We report the results of CCD V and r photometry of the globular cluster NGC 5466. The difference image analysis technique adopted in this work has resulted in accurate time-series photometry even in crowded regions of the cluster enabling us to discover five probably semiregular variables. We present new photometry of three previously known eclipsing binaries and six SX Phe stars. The light curves of the RR Lyrae stars have been decomposed in their Fourier harmonics and their fundamental physical parameters have been estimated using semi-empirical calibrations. The zero-points of the metallicity, luminosity and temperature scales are discussed and our Fourier results are transformed accordingly. The average iron abundance and distance to the Sun derived from individual RR Lyrae stars, indicate values of [Fe/H]=−1.91 ± 0.19 and D= 16.0 ± 0.6 kpc, or a true distance modulus of 16.02 ± 0.09 mag, for the parent cluster. These values are, respectively, in the Zinn & West metallicity scale and in agreement with recent luminosity determinations for the RR Lyrae stars in the Large Magellanic Cloud. The MV–[Fe/H] relation has been recalibrated as MV=+(0.18 ± 0.03)[Fe/H]+ (0.85 ± 0.05) using the mean values derived by the Fourier technique on RR Lyrae stars in a family of clusters. This equation predicts MV= 0.58 mag for [Fe/H]=−1.5, in agreement with the average absolute magnitude of RR Lyrae stars calculated from several independent methods. The MV–[Fe/H] relationship and the value of [Fe/H] have implications on the age of the globular clusters when determined from the magnitude difference between the horizontal branch and the turn-off point (HB-TO method). The above results however would not imply a change in the age of NGC 5466, of 12.5 ± 0.9 Gyr, estimated from recent isochrone fitting.