S-adenosylmethionine and morphogenesis inMucor racemosus

Gene expression inMucordimorphism

An ongoing dialectic has concerned the relative importance of differential gene expression versus the pattern of new wall deposition in Mucor dimorphism. Numerous physiological processes and enzyme activities have been observed in flux during morphogenesis, but a causal link to dimorphism has been infrequently demonstrated. Very few of the proteins that are conspicuous in two-dimensional polyacrylamide gel electrophoresis are specific to cell morphology or significantly change in amount during morphogenesis. Cyclic AMP, putrescine, S-adenosylmethionine, and enzymes governing their intracellular concentrations show patterns of change that consistently correlate with morphogenesis. The expression of RAS proteins and translation elongation factor-1α activity during morphogenesis are regulated at the level of transcription and post-translational methylation, respectively. Wall chemistry is very similar in both morphologies, but wall deposition is isodiametric in yeasts and vectorial in hyphae. Electron microscopy shows patterns of apparent exocytosis that are generalized in the former and apical in the latter. Research on other dimorphic fungi, including Saccharomyces cerevisiae, suggests an involvement of cytoskeletal proteins and a family of GTP-linked protein kinases in directing polar growth. Some of these elements, which may be controlled quite distal from the genes encoding them, have been demonstrated in Mucor spp., while others are the subject of ongoing investigations. Key words: Mucor, dimorphism, morphogenesis, gene expression, yeasts, hyphae.

Polyamines, DNA methylation, and fungal differentiation

Mucorales constitute a group of fungi that, because of their growth characteristics, have been used extensively in the study of cell differentiation, cell morphogenesis, and stimuli perception. We have studied the role of polyamine metabolism in the development of different Mucorales, with emphasis on Mucor and Phycomyces species. It has been observed that previous to each differentiative step, the cellular levels of the most regulated enzyme of the pathway, ornithine decarboxylase (ODC), and polyamines suffer a noticeable increase. Addition of diaminobutanone (DAB), a competitive inhibitor of ODC, blocks all the corresponding differentiative phenomena. In its presence, germinating spores fail to produce germ tubes and keep growing isodiametrically; mycelia do not sporulate but continue their vegetative growth, and yeast cells are unable to engage in a dimorphic transition without alterations in their growth rate. This differential effect of the ODC inhibitor in growth and development is apparently due to the location of ODC in at least two different cell compartments, one of which is impermeable to the drug. Inhibition of development is counteracted by putrescine and more noticeably by 5-azacytidine (5AC), a strong inhibitor of DNA methylation. Methylation levels of DNA are high in spores, and they become reduced after germination. Demethylation is inhibited by hydroxyurea, which blocks DNA replication, and by DAB. The effect of the latter is reversed by 5AC. These results suggest a relationship between polyamines and DNA methylation. Analysis of metallothioneine gene (CUP) behavior and expression during spore germination has confirmed this hypothesis.(ABSTRACT TRUNCATED AT 250 WORDS)