The fundamental activities of the replicative primosomes of Escherichia coli are provided by DnaB, the replication fork DNA helicase, and DnaG, the Okazaki fragment primase. As we have demonstrated previously, DnaG is recruited to the replication fork via a transient protein-protein interaction with DnaB. Here, using sitedirected amino acid mutagenesis, we have defined the region on DnaB required for this protein-protein interaction. Mutations in this region of DnaB affect the DnaB-DnaG interaction during both general primingdirected and X174 complementary strand DNA synthesis, as well as at replication forks reconstituted in rolling circle DNA replication reactions. The behavior of the purified mutant DnaB proteins in the various replication systems suggests that access to the DnaG binding pocket on DnaB may be restricted at the replication fork.

Levitation and rotation of cylindrical rare-earth magnets on yttrium-barium-copper-oxide superconducting bearings has been sustained at speeds of up to 120 000 rpm with a surface speed of 40 m/s. The decay of the free rotation rate has been measured at both atmospheric pressure and a partial vacuum to 2.6 μm Hg. The decay measurements in vacuum indicate that the flux drag torques are constant and independent of speed. The magnetic shear stress on the rotor magnet is estimated to be 150 dyn/cm2. It is believed that drag torques on rotating magnets are related to asymmetry in the flux density pattern of the magnet.

Levitation forces between a small permanent magnet and a disk of bulk high-temperature superconductor at 77 K were measured as a function of vertical separation for disks of composition Y-Ba-Cu-O, Ag/Y-Ba-Cu-O, (Pb,Bi)-Sr-Ca-Cu-O, and Tl-Ba-Ca-Cu-O. The forces were highly hysteretic; however, for all samples, on the initial descent of the magnet toward the disk, the force was unique, independent of magnet speed, and varied approximately as the negative exponential of the separation distance. Magnetic stiffness, associated with minor hysteresis loops, was found to be approximately proportional to the levitation force, and nearly independent of magnet configuration and superconductor composition.

Measurements of levitation forces on melt-quenched and free sintered YBa2Cu3O x superconductors show that melt-quenched samples produce forces from 96-170% higher than sintered specimens. The force increases are correlated with higher magnetization values for the melt-quenched material. Similar increases in the magnetic stiffness were also measured. Vertical levitation force relaxation showed less than 5% decrease in one hour. These results have important favorable implications for application to superconducting bearings.

Dynamic forces between small permanent magnets and bulk high-Tc superconducting ceramic materials have been measured, including magnetic stiffness and damping effects. The vibration frequencies and the related magnetic stiffness show a large dependence on the magnet-superconductor distance. The dynamic magnetic stiffness is shown to be related to small static reversible magnetic forces, but is not correlated with the static hysteretic forces. The magnetic damping is inversely related to the magnet-superconductor distance and can critically damp the oscillations at small gaps. These data also show that the flux pinning forces depend on the prior flux history in the superconductor.