R-loop and restriction mapping procedures reveal the organization of coding regions at each end of the giant rDNA palindrome of Physarum polycephalum. A 19S coding region of 2.10±0.21 kb is located at each end of a very long central spacer (35.64±2.08 kb). An Internal spacer of 1.66±0.12 kb lies distal to the 19S gene. The 5.8S rRNA coding region 1s located in this spacer. The 26S gene lies distal to the internal spacer. The 26S gene is unusual among those of eukaryotes in that it consists of 3 coding regions (α, β and γ) interrupted by 2 intervening sequences. The 26S α (most central) coding segment of 2.41±0.33 kb 1s separated from the 26S β segment by an intervening sequence of 0.68±0.13 kb. The 26S β segment (0.70±0.11 kb) is separated from the most distal 26S γ segment (0.59±0.14 kb) by an intervening sequence of 1.21±0.14 kb. The 2 intervening sequences are present in at least 88% of ribosomal genes from active plasmodia, Indicating that genes containing these sequences are transcribed. The rDNA termini contain a heterogeneous region which varies in length by±300 base pairs.
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