Meiosis in Schizophyllum commune: the effect of hydroxyurea on basidiospore sporulation, germination, and nuclear number

Ribonucleotide reductases and their occurrence in microorganisms: a link to the RNA/DNA transition

The evolution of a deoxyribonucleotide synthesizing ribonucleotide reductase might have initiated the transition from the ancient RNA world into the prevailing DNA world. At least five classes of ribonucleotide reductases have evolved. The ancient enzyme has not been identified. A reconstruction of the first ribonucleotide reductase requires knowledge of contemporary enzymes and of microbial evolution. Experimental work on the former focuses on few organisms, whereas the latter is now well understood on the basis of ribosomal RNA sequences. Deoxyribonucleotide formation has not been investigated in many evolutionary important microorganisms. This review covers our knowledge on deoxyribonucleotide synthesis in microorganisms and the distribution of ribonucleotide reductases in nature. Ecological constraints on enzyme evolution and knowledge deficiencies emerge from complete coverage of the phylogenetic groups.

Meiosis in Schizophyllum commune: The effect of hydroxyurea on the frequency of recombination and mutations

The effect of exposure of nuclei at different stages of meiosis to hydroxyurea (HU) was tested by examination of its effect on the frequency of auxotrophic mutations and on intergenic recombination. The results indicate that recombination is increased most significantly if the nuclei are exposed to the drug during the premeiotic-S phase and to a lesser extent if exposed during prophase. Similarly, nuclei exposed to HU during premeiotic-S phase and prophase mutate at a frequency 10 to 50 times higher than untreated meiotic nuclei. The degree of response of the mutagenic events and recombination events is not similar and it is suggested that the mutagenic events occur at stages prior to recombination. The temporal relations of the two events in response to HU suggest that the mutagenic events precede recombination and the results obtained with HU only amplify the mutagenic events that normally occur in meiotic nuclei and are related to the meiotic effect.

The effect of hydroxyurea on meiosis and genetic recombination in the fungus Coprinus lagopus

Treatment with hydroxyurea (HU) inhibits fruitbody development in Coprinus lagopus (sensu Lewis). The two most sensitive meiotic stages are mid-late premeiotic S phase and the pachytene-diplotene period. A 2 or 4 h treatment at mid-late premeiotic S phase arrests fruitbody development. If the same treatment is given at pachytene and diplotene, the fruitbody completes meiosis but the spores produced are inviable. The spores from fruitbodies treated at pachytene appear to be normal whereas those treated at diplotene are empty because the four nuclei remain in the basidium. A 2 or 4 h treatment with HU during karyogamy causes a decrease in spore viability and a dramatic increase in recombination frequency. The same treatment at metaphase II or later stages causes little damage to fruitbody development or spore viability. The effects of HU on the meiotic cell cycle suggest that the drug exerts its effect by inhibiting the synthesis of deoxynucleotides.

Meiosis in Schizophyllum commune: premeiotic DNA replication and meiotic synchrony induced with hydroxyurea

Hydroxyurea (HU) effectively inhibits meiosis inSchizophyllum. The predominant cytological stage in inhibited fruit bodies is fusion. The inhibition is reversible and makes possible synchronization of a naturally nonsynchronous system. Microphotometric determinations of the DNA content in prefusion nuclei treated with HU suggest that premeiotic DNA replication occurs in prefusion nuclei. Synaptinemal complexes are not completed in HU-treated nuclei, suggesting that this event is dependent on premeiotic DNA replication.