[Studies on development of the phycomycete Allomyces arbuscula. III. Polysome formation and RNA metabolism during differentiation of gametangia (author's transl)].

Polysomes were isolated both from growing gametophytes of Allomyces arbuscula and from gametangia prepared from mycelia at different periods during gametogenesis. Analysis of polysomes by sucrose gradients showed that ribosomes present in the gametangia monosome pool were shifted into polysomes. This shift was found to be correlated with gametangia differentiation. The ribosome distribution remained virtually unchanged during the early stage of gamete formation. In mature gametes and swarming zygotes a low level of polysomes was detected. Labeling of rRNA by 32PO4 demonstrated a de novo synthesis of monosomes throughout the period of gametangia differentiation. No incorporation of 32PO4 was found to be present in ribosomes prepared from gametangia after onset of gamete formation. On the basis of these labeling experiments it is concluded that radioactivity in polysomes extracted from mature gametes and swarming zygotes can be attributed in part to conserved mRNA. Synchronous formation of gametangia was induced by transferring the vegetative mycelia from growth medium into a low salt buffer. Under these conditions the incorporation of either 32PO4 or 3H-uridine into RNA, particularly into rRNA, was found to be markedly decreased. This obviously indicates a shutdown of RNA synthesis. rRNA from induced mycelia examined by polyacrylamide gel electrophoresis was found to be severely degraded. In contrast to this, rRNA isolated from ribosomes of developing gametangia and from gametes exhibited no degradation products. It is suggested that endonucleases cause rRNA hydrolysis in the hyphal cytoplasm during gametangia differentiation. Ribosomes compartmentalized in gametangia seem to be inaccessible to nucleases during the later process for gametogenesis.