Human ribosomal DNA: conserved sequence elements in a 4.3-kb region downstream from the transcription unit

The sequence of 4366 bp of nontranscribed spacer (NTS) human ribosomal DNA (rDNA) located downstream from the 3' end of the transcription unit has been determined. The NTS rDNA is rich in pyrimidine nucleotides (31% T and 30% C) that tend to occur on the coding strand in runs of simple sequence repeats. Other highly repetitive sequence elements are also represented, including tracts of (dA-dC)26 and (dG-dT)29 on the coding strand downstream from the putative termination of transcription. Still farther downstream, two Alu repeat sequences are found. Such sequences are also found in rat DNA at comparable locations, consistent with the possibility of a comparable functional role.