Ribosomal genes in Physarum polycephalum: transcribed and non-transcribed sequences have similar base compositions

The transcribed and non-transcribed sequences in Physarum polycephalum ribosomal DNA (rDNA) were separated by restriction nuclease digestion of pure rDNA and the products fractionated by zone sedimentation in sucrose gradients. The base compositions of the fragments were determined by analytical centrifugation in CsCl or in CsCl with netropsin. All the fragments had dA + dT contents in the range 44-48%. From the known sequence arrangement and transcription pattern of Physarum rDNA it was concluded that coding sequences, transcribed but non-coding sequences, and non-transcribed sequences all possess similar base compositions, contrary to the situation in many other systems. The thermal denaturation profile of Physarum rDNA is reported. It suggests the rDNA sequence is complex and supports the above conclusion of limited heterogeneity of base composition.