THE SUPPRESSION OF GENE EXPRESSION BY ALPHOID REPETITIVE-DNAS FROM PRIMATES

Abstract

Primate genomes contain abundant repeated sequences termed alphoid DNAs which consist of tandem repeats of about 170 bp and are distributed among all chromosomes. To elucidate the unknown function of mammalian highly repeated DNA, both human and monkey alphoid DNAs were cloned into vectors that contain the SV40 ori, and SV40 promoter-enhancer regions driving the chloramphenicol acetyl transferase (CAT) gene. The alphoid sequences suppress CAT expression when the plasmids were transfected into the CV-1 monkey cell line. Suppression depended strongly on the orientation and the position of the alphoid DNA inserts in the vectors. Only one orientation suppressed the gene efficiently. The alphoid sequences suppressed the gene if they were located 3{dollar}\sp\prime{dollar} of the CAT gene, about 2 kb from the SV40 enhancer. There was little suppression and even stimulation of CAT gene expression when the alphoid sequences were located 5{dollar}\sp\prime{dollar} to the gene, about 300 bp from the SV40 enhancer.;Southern and Northern blots indicated that the suppression was at the RNA level. In CV-1 cells the transfected plasmids were not replicated: replication ensued if the plasmids were cotransfected with a vector expressing the SV40 T-antigen or if the plasmids were introduced into SV40-transformed COS-7 cells. Under these conditions, the alphoid suppression was overcome. However, the replication function of COS-7 or T antigen expression was not involved in overcoming the suppression by alphoid DNA sequences. Linearized forms of the plasmids exhibited only one-tenth the activity of the closed circular forms with the same intranuclear survival rate. Competition expression experiments with alphoid sequences showed that the suppression is cis -acting, and could be released by high dosages of alphoid sequences in trans. This suggested that alphoid DNAs bind specific inhibitors of gene expression in cis, and that the inhibitors could be titrated by high dosages of alphoid sequences in trans.