Locations and contexts of sequences that hybridize to poly(dG-dT)·(dC-dA) in mammalian ribosomal DNAs and two X-linked genes

Sequences located several kilobases both 5 ' and 31 of the stably transcribed portion of several genes hybridize to radio-labeled pure fragments of the alternating sequence poly (d6-dT)(dC-dA) ['poly(GT)"]. The genes Include the ribosomal DNA of mouse, rat, and human, and also human glucose-6-phosphate dehydrogenase (G6PD) and mouse hypoxanthine-guanine phosphoribosyl transferase (HPRT). HPRT has additional hybridizing sequences 1n Introns. Fragments that Include the hybridizing sequences and up to 300 bp of adjoining DNA show perfect runs of poly(ST) (>30bp) 1n all but the human 5' region of rDNA, which shows a somewhat different alternating pur1ne:pyr1m1d1ne sequence, poly(GTAT) (36bp). Within 150 bp of these sequences 1n various Instances are found a number of other sequences reported to affect DNA conformation 1n model systems. Host marked 1s an enhancement of sequences matching at least 67X to the consensus binding sequence for topoisomerase II. Two to ten-fold less of such sequences were found 1n other sequenced portions of the nontranscribed spacer or 1n the transcribed portion of rDNA. The conservation of the locations of tracts of alternating pur1ne:pyr1ra1d1ne between evolutionarily diverse species 1s consistent with a possible functional role for these sequences