We have used the polymerase chain reaction to isolate fragments of Ty1-copia group retrotransposons from a wide variety of members of the higher plant kingdom. 56 out of 57 species tested generate an amplified fragment of the size expected for reverse transcriptase fragments of Ty1-copia group retrotransposons. Sequence analysis of subclones shows that the PCR fragments display varying degrees of sequence heterogeneity. Sequence heterogeneity therefore seems a general property of Ty1-copia group retrotransposons of higher plants, in contrast to the limited diversity seen in retrotransposons of Saccharomyces cerevisiae and Drosophila melanogaster. Phylogenetic analysis of all these sequences shows, with some significant exceptions, that the degree of sequence divergence in the retrotransposon populations between any pair of species is proportional to the evolutionary distance between those species. This implies that sequence divergence during vertical transmission of Ty1-copia group retrotransposons within plant lineages has been a major factor in the evolution of Ty1-copia group retrotransposons in higher plants. Additionally, we suggest that horizontal transmission of this transposon group between different species has also played a role in this process.

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The electronic structure of the layered compound 1T-TiTe 2 has been studied in detail by high-resolution angle-resolved photoelectron spectroscopy ͑ARPES͒ and density-functional band calculations. The results confirm the semimetallic nature of this material as due to an overlap of Te 5p-and Ti 3d-like conduction bands. We find an overall good correspondence between experiment and theory, with all ARPES structures accounted for by the calculated band structure. Particular focus is applied to the bands near the Fermi level and to the Fermi-surface topology. Interesting behavior is observed for an essentially Ti 3d Z 2 -derived conduction band, whose measured Fermi vector and qualitative shape are excellently reproduced by the calculation. However, the experimental energy dispersion of the Ti 3d z 2 ARPES peak appears to be considerably reduced with respect to band theory. From these results we obtain a picture of the electronic structure of 1T-TiTe 2 as that of a Fermi liquid with renormalized quasiparticle dispersions and a Fermi surface in accordance with Luttinger's sumrule. We show that the experimental Ti 3d z 2 emission is quasi-two-dimensional near the Fermi surface, which, together with its being remarkably unobscured, virtually free of any interference with other spectral structures or inelastic background, makes it an ideal object for ARPES line-shape studies on a Fermiliquid system.

The electronic structures of single-crystal and ceramic samples of the electron-doped superconductor Nd2-xCe x Cu04->-with x-0 and 0.15 have been studied using resonant photoemission spectroscopy. Comparing these spectra with previous results for La2-xSr x CuC>4 shows that for both electron and hole doping the Fermi level lies at nearly the same energy, in states that fill in the x x 0 insulator gap. We point out that this would occur if the states at the Fermi level obey a Luttinger-type sum rule, i.e., according to standard theory, if the metallic state is a normal Fermi liquid.