Synthesis of polyamines in the germinating conidia of Aspergillus nidulans.

In a number of biological systems undergoing accelerated growth there is a rapid accumulation of polyamines, particularly spermidine, in parallel with RNA. One of the first events in such systems is a rapid increase in the activity of ornithine decarboxylase to as much as 100 times control values (Williams-Ashman, 1972). However, the increase in the amount of polyamines is much less marked, usually less than tenfold. The ratelimiting steps in polyamine biosynthesis are believed to be catalysed by ornithine decarboxylase (EC 4.1.1.17) or S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50). During the germination of conidia of Aspergillus nidulans both these enzymes undergo a similar increase to a maximum total activity and specific activity at 8-10h (Stevens et al. 1976), whereas the total protein per culture continues to increase for much longer. Further, the ornithine decarboxylase in A. nidulans is strongly inhibited unless the extracts are fist dialysed, and thus the physiological significance of the ornithine decarboxylase measured in vitro is unclear. To clarify this we measured the amounts of putrescine, spermidine and spermine and the incorporation in vivo of ornithine into the polyamines. Conidia of A. nidulans (BWB 272) were grown as described previously (Stevens et al., 1976). Ornithine and polyamines were extracted in 2 % (w/v) HCIOI and separated on aDowex50U'column (1 cmx4cm).Thecolumn was washedwith20mlof0.5~-HCI,and ornithine and polyamines were eluted with 15ml of ~ M H C ~ . The concentrated extract, dissolved in O.O~M-HCI, was used for determination of ornithine by paper electrophoresis (Evered, 1959) and of polyamine both by the paper-electrophoretic method (Raina & Cohen, 1966) and by t.1.c. of the dansylated derivatives (Seiler & Weichmann, 1967). Good agreement was obtained between both methods. For measurement of polyamine synthesis, 20,uCi of [3H]ornithine (5.0Ci/mmol) was added to the submerged cultures at various times and they were harvested 1 h later. The amounts of putrescine, spermidine and spermine are shown in Fig. 1. Spermidine is the predominant polyamine. The putrescine concentration remains low throughout