No abstract

The effect of the strong promoter from the alcohol dehydrogenase gene on mitotic and meiotic intragenic recombination has been studied at the ade6 locus of the fission yeast Schizosaccharomyces pombe. A 700-bp fragment containing the functional adhi promoter was used to replace the weak wild-type promoter of the ade6 gene. Analysis of mRNA showed that strains with this ade6::adhl fusion construct had strongly elevated ade6-specific mRNA levels during vegetative growth as well as in meiosis. These increased levels of mRNA correlated with a 20-to 25-fold stimulation of intragenic recombination in meiosis and a 7-fold increased prototroph formation during vegetative growth. Analysis of flanking marker configurations of prototrophic recombinants indicated that simple conversions as well as conversions associated with crossing over were stimulated in meiosis. The strongest stimulation of recombination was observed when the adhl promoter was homozygous. Studies with heterologous promoter configurations revealed that the highly transcribed allele was the preferred acceptor of genetic information. The effect of the recombinational hot spot mutation ade6-M26 was also investigated in this system. Its effect was only partly additive to the elevated recombination rate generated by the ade6::adhl fusion construct.The mechanisms of genetic recombination have been investigated in great detail in the last few years. Although much information has been collected and several models for recombination have been proposed (e.g., references 26, 27, 31, 32, and 40), the mechanisms are not yet fully understood. However, it has become clear that several aspects of DNA metabolism, such as DNA repair and transcription, are closely interrelated with recombination.Transcription and recombination studies have been done particularly in eucaryotes. The results show that recombination between segments of the immunoglobulin heavy-chain locus is accompanied by transcription. A role in targeting of the recombination events has been proposed for this activity (3,46,47). In addition, recombination between adenovirus DNA and hamster, mouse, or human cell DNA is also found to occur at transcriptionally active regions. It was proposed that transcription establishes a chromatin configuration which is more accessible to foreign DNA and for the recombination machinery (35).In the budding yeast Saccharomyces cerevisiae, several systems that show a correlation between transcription and recombination have been reported. Mating-type switching occurs via a double-strand break which is induced by the HO endonuclease only in the expressed cassette of the matingtype locus (38). However, if the two remaining mating-type cassettes that are normally transcriptionally silent become expressed as the result of a mutation in one of the four SIR genes (18, 21), they too serve as recipients for the HOinduced double-strand cut (20). Recently it has been shown that the SIR2 gene not only is involved in the control of expression of the mating-type system but also represses bot...

No abstract

Photoelectron-spin-polarization measurements with photon energies up to 11 eV on Fe,04 and energy distribution curves in the photon energy range 5 &. 'lv& 90 eV on magnetite (Fe,04) and wustiie (FeO) are interpreted in terms of the atomic theory of the single-ion-in-a-crystal-field model. The combination of the two different experiments yields the shapes and positions of the filled oxygen 2p bands and the 3d" ' final states of Fe ions with a reliability previously not attained. The 3d" '-multiplet structure of Fe304 can be explained with the following set of parameters: 10Dq = 1.75 eV and the Racah parameter 8 = 645 cm ' for Fe'+ left behind in 8 lattice sites; 10Dq = 1.55 eV for Fe'+ in A sites. The difference in threshold for photoionization of Fe'+ and Fe'+ in 8 sites is 1.0 eV. The oxygen 2p emission is found to be centered at 7.3 eV below the Fermi level with a full width of 3 eV at half-maximum. The 3d-multiplet structure of Fe,O can be explained with 10Dq = 1.7~0.1 eV and 8 = 800 cm

No abstract