BINDING OF NUCLEAR FACTORS TO A SATELLITE DNA OF RETROVIRAL ORIGIN WITH MARKED DIFFERENCES IN COPY NUMBER AMONG SPECIES OF THE RODENT CTENOMYS

The major satellite DNA of the subterranean rodent Ctenomys, named RPC S, contains several consensus sequences characteristic of the U3 regio n of retroviral long terminal repeats (LTRs), such as a polypurine tra ct, CCAAT boxes, binding sites for the CCAAT/enhancer-binding protein (C/EBP), a TATA box and putative polyadenylation signals. RPCS present s an enormous variation in abundance between species of the same genus : while C.australis or C.talarum have approximately 3x10(6) copies per genome, C.opimus has none. A sequence (RPCS-I) with identity to the S V40-enhancer core element, present in all the repeating units of the s atellite is specifically protected in DNase I footprintings. Competiti ons of band-shift assays with different transcription factor binding s ites indicate that binding to RPCS-I is specific and involves CCAAT pr oteins related to NF-1, but not to C/EBP. By the use of quantitative p rotein/DNA binding assays we determined that, despite of their conspic uous difference in RPCS copy number, C.talarum and C.opimos have equiv alent amounts and identical quality of RPCS-binding proteins. These re sults are consistent with the observation, by in situ hybridization, t hat RPCS is clustered in heterochromatic regions, where it might have restricted accessibility to transcription factors in vivo. This is the first report of the binding of transcription factors to a satellite D NA of retroviral origin.