In order to elucidate the mechanisms of fluoroquinolone resistance in Enterococcus faecium, spontaneous mutants isolated from Ent. faecium ATCC 19434 by stepwise selection with sparfloxacin (SPX) or norfloxacin (NOR) and 13 clinical isolates of Ent. faecium were characterized by analysing quinolone-resistance-determining regions (QRDRs) of the gyrA, gyrB, parC and parE genes and examining changes in MICs of SPX and NOR in the presence of efflux pump inhibitors. The SPX-selected first-step mutant had a point mutation only in gyrA, and the mutants QR7-18 and QR7-39, and clinical isolates that had point mutations in parC, showed NOR resistance. These results indicate that the primary targets of SPX and NOR are DNA gyrase and topoisomerase IV, respectively, and therefore that the primary target of fluoroquinolones in Ent. faecium differs depending on the structure of the compound used. The characterization of the spontaneous mutants and the clinical isolates demonstrates that in addition to the previously reported alterations in GyrA and ParC, an alteration in GyrB, a NorA-like pump, an unknown efflux pump, which excretes both SPX and NOR from bacterial cells, and probably other unknown mechanism(s) all contribute to fluoroquinolone resistance in Ent. faecium.

The active centers of NiFe hydrogenase from Desulfovibrio vulgaris Miyazaki F have been located in the electron-density map calculated at 4 A resolution. The electron-density map based on five heavy-atom derivatives showed four strong peaks which were clearly distinguished from the protein region. These strong densities have been successfully assigned to three iron-sulfur clusters and one Ni atom by a difference Fourier technique with coefficients of the best phases from the multiple isomorphous replacement (MIR) method and structure factors obtained at five wavelengths (1.040, 1.487, 1.730, 1.743 and 1.750 A) with the use of a synchrotron radiation source. Four active centers are approximately lined up at a distance of ca 13 .~, which seems reasonable if they are connected with the electron-transfer chain.

Fused ring systems, containing two or three five-membered rings, were constructed by utilizing 1 -(cyclopent-1 -enylcarbonyl)vinylphosphonates which function as versatile annulating reagents.

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