Magnetic shape-memory effects in a crystal

Magnetic fields affect the motion of electrons and the orientation of spins in solids, but are thought to have little impact on crystal structure, particularly in compounds with low magnetic susceptibility, such as antiferromagnets. Here we describe an unexpected magnetic effect on crystal shape, in which the direction of the crystal's axes are swapped and the shape changes when a magnetic field is applied; this in turn induces curious memory effects in resistivity and magnetic susceptibility. Ironically, this phenomenon occurs in one of the most well-studied two-dimensional antiferromagnets, La(2-x)Sr(x)CuO(4).     

Nonspecific integration of the HTLV provirus genome into adult T-cell leukaemia cells

Human T-cell leukaemia virus (HTLV), previously also reported as ATLV, is a recently identified retrovirus which is closely associated with adult T-cell leukaemia (ATL) endemic in southwestern Japan and the Caribbean. Determination of the total nucleotide sequence of the HTLV genome has revealed no typical onc gene acquired from the cellular sequence. Screening of the HTLV provirus genome in tumour cells has shown that in all cases of ATL examined, the primary tumour cells contained the provirus genome and were monoclonal with respect to the integration site of the provirus. These findings suggest that ATL leukaemogenesis may be due to insertional mutagenesis in which the provirus genome is integrated into a specific locus on the chromosomal DNA and then activates an adjacent cellular onc gene, a mechanism already demonstrated in avian lymphoma and erythroblastosis induced by avian leukosis viruses. A common site of HTLV provirus integration in leukaemic cells among some ATL patients was reported by Hahn et al. but subsequently retracted. However, this retraction does not imply the random integration of the proviruses. Independently, we have been testing this insertional mutagenesis model in ATL and report here that the provirus did not have a common locus of integration in 35 ATL patients and did not integrate on the same chromosome in 2 ATL patients.     

Effect of novobiocin on initiation of DNA replication in Bacillus subtilis.

The initiation of DNA replication of small replicons in vitro involves conformational changes in the whole DNA molecule or in the region near to the replication origin. One striking finding has been the role of DNA gyrase (that is, the necessity for supercoiled structure) in the initial stage of ColE1 replication in vitro. However, little is known about the effect of gyrase on the initiation of replication of bacterial chromosomes in vivo. We have constructed a map of cleavage sites of restriction enzymes at the region of the origin of replication of the Bacillus subtilis chromosome (accompanying paper). This has now enabled us to examine the effect of novobiocin, a selective inhibitor of DNA gyrase, on the replication of the specific chromosomal segments near the origin and to seek a possible role for the gyrase in the initiation of chromosomal replication. We have found that only a limited segment of the chromosome at the origin region was replicated in the presence of novobiocin. This effect allowed us to locate the site of the origin of replication to within a DNA fragment of molecular weight 3.4 x 10(6).     

A universal scaling relation in high-temperature superconductors

Since the discovery of superconductivity at elevated temperatures in the copper oxide materials there has been a considerable effort to find universal trends and correlations amongst physical quantities, as a clue to the origin of the superconductivity. One of the earliest patterns that emerged was the linear scaling of the superfluid density (rho(s)) with the superconducting transition temperature (T(c)), which marks the onset of phase coherence. This is referred to as the Uemura relation, and it works reasonably well for the underdoped materials. It does not, however, describe optimally doped (where T(c) is a maximum) or overdoped materials. Similarly, an attempt to scale the superfluid density with the d.c. conductivity (sigma(dc)) was only partially successful. Here we report a simple scaling relation (rho(s) proportional, variant sigma(dc)T(c), with sigma(dc) measured at approximately T(c)) that holds for all tested high-T(c) materials. It holds regardless of doping level, nature of dopant (electrons versus holes), crystal structure and type of disorder, and direction (parallel or perpendicular to the copper-oxygen planes).