Ribosomal RNA genes in the amoebal and plasmodial forms of the slime mould Physarum polycephalum.

1. The degree of homology between ribosomal RNA isolated from microplasmodia and amoebae of the slime mould Physarum polycephalum has been determined by competive hybridisation of the RNA from the two sources to homologous DNA in solution. The extent of competition was measured both by hybridisation to saturation and by following the kinetics of hybrid formation. In each case competition was found to be 100%, indicating that the ribosomal RNAs from the two, quite different vegetative forms of the organism exhibit a high degree of homology and are probably transcribed from the same genes. 2. The relationship between the amount of nuclear DNA that codes for ribosomal RNA (rDNA) and ploidy has been investigated in three strains of P. polycephalum which exhibit a 1:2:5 variation in the amount of DNA per nucleus. Ribosomal RNA saturation values were determined by hybridisation to DNA isolated from prophase nuclei of plasmodia. The proportion of rDNA was found to be constant at 0.16--0.18% of the total genome in the three strains.