Transcription of human ribosomal DNA may terminate at multiple sites.

The termination of human pre-rRNA transcription has been investigated. The most abundant possible termination site was detected 360 bp downstream from the 28S gene, in front of the first SalI box of the rDNA spacer. This site, however, is partially bypassed during transcription, and three additional termination points were detected inside the heterogeneous region of the rDNA spacer. Later sites were mapped about 930, 1030 and 1110 bp downstream from the 3' end of the 28S rRNA gene. The authors suggest that the T clusters and pyrimidine-rich regions play an important role in the termination processes. They either may influence the efficiency of the SalI boxes in terminating the synthesis of pre-rRNAs or may serve as independent signals for the fail-safe termination of readthrough transcripts. In both cases transcription of human rDNA ceases at multiple sites.