Unequal accumulation of 26S and 17S RNAs in ribosomes during spore germination in Dictyostelium discoideum

Ribosome synthesis was studied in spores at the swelling stage and compared with freshly emerged and logarithmically growing vegetative amoebae. During the swelling stage of spore germination, ribosome synthesis was abnormal. Newly made ribosomes accumulated unequal amounts of 26S and 17S rRNAs. The stoichiometric ratio 26S:17S was 0.5 in swelling spores, compared with 0.9 in amoebae. The relative level of pre-rRNA persisting in the nucleus was apparently 2- to 3-fold higher in swelling spores than in amoebae. All of the known ribosomal proteins, except for a few, were made during the swelling stage and were associated with the newly made ribosomes in expected amounts. Analysis of the 2'-O-methyl ribose content in the newly made rRNAs suggest that methylation was defective in swelling spores. Compared with growing amoebae, the methyl content was 30 and 64% less in 26S and 17S RNAs from the swelling stage, respectively. It is suggested that undermethylation could be partly responsible for the differential accumulation of newly made 26S and 17S RNAs during the early stages of spore germination in Dictyostelium discoideum.