The role of polyamines in fungal cell differentiation.

Polyamines are low molecular weight polycations which are present in all organisms, both procaryotic and eucaryotic (1). The most widely distributed polyamines are putrescine, spermidine and spermine; nevertheless, a large number of fungal species are devoid of spermine. Polyamines are essential for growth, and mutants affected in their synthesis become auxotrophic. Regarding their physiological roles, it has been demonstrated that polyamine starvation leads to reduction in the synthesis of nucleic acids and proteins. It has been concluded that polyamines are essential for macromolecule synthesis, although their precise mode of action remains unknown (2) (Table 1). Because of their net charge, it has been suggested that polyamines bind to macromolecular anions such as nucleic acids and phospholipids, stabilizing their structure. Levels of polyamines, as well as those of the first enzyme in the biosynthetic route: ornithine decarboxylase (ODC), increase during the phases of active growth and differentiation in distinct eucaryotic systems. In fungi the role of polyamines in cell differentiation remains debatable since no clear cut correlation between their levels and development has been demonstrated. This lack of correlation may be due to the fact that most polyamines present in the cell are inside the vacuole or bound to all polyanions, only a small amount remaining free to fulfill other tasks associated with development (3).