The tetracycline resistance gene tet(M) exhibits mosaic structure.

Tetracycline resistance genes of the M class, tet(M), are typically found on mobile genetic elements as the conjugative transposons of gram-positive bacteria. By comparing the sequences of eight different tet(M) genes (from Enterococcus faecalis, Streptococcus pneumoniae, Staphylococcus aureus, Ureaplasma urealyticum, and Neisseria), a mosaic structure was detected which could be traced to two distinct alleles. The two alleles displayed a divergence of 8% and a different G/C content. The block structure of these genes provides evidence for the contribution of homologous recombination to the evolution and the heterogeneity of the tet(M) locus. Unlike described cases of chromosomally located mosaic loci, tet(M) is a relatively recently acquired determinant in the species examined and it would appear that mosaic structure within tet(M) has evolved after acquisition of the gene by the mobile genetic elements upon which it is located

Theoretical investigation of free-standing CoPd nanoclusters as a function of cluster size and stoichiometry in the Pd-rich phase: Geometry, chemical order, magnetism, and metallic behavior

We report on a study of the geometrical structure, magnetic properties, and metallic behavior of free-standing CoPd nanoclusters with N=7, 13, 19, 23, and 26 atoms, as a function of cluster size and stoichiometry, in the Pd-rich phase. The investigated structures were obtained using a Gupta potential in combination with a genetic algorithm energy minimization. The electronic properties of the ground state geometrical structures were determined by solving a self-consistent spd tight-binding Hamiltonian. The metallic behavior was studied using Kubo's criterion. We tested our approach against benchmark SIESTA calculations for some of the small clusters investigated in the present work. We discuss the competition between segregation and mixing effects by means of an order parameter. The magnetic moments are analyzed in the context of the local chemical and atomic environments and we propose a simple empirical expression for the average magnetic moment per atom of these binary clusters. ©