A human B-box-binding protein downregulated in adenovirus 5-transformed human cells.

Internal promoters of some genes transcribed by RNA polymerase III (e.g. tRNA genes, adenovirus VA1 RNA gene, human retroposons of the Alu family) contain a conserved sequence element, B-box, interacting with basal transcription factor TFIIIC2 which initiates assembly of the full transcription complex on the genes, and which represents the major determinant of the efficiency of their expression. In this study we have identified in human nuclear extracts a protein which interacts with VA1 B-box DNA and forms a high-affinity complex which is very stable after the addition of a large excess of competitor DNA. Unlike TFIIIC2, the B-box-binding activity of the B-box-binding protein is found to be decreased in adenovirus 5-transformed human cells. In these cells (line 293) increased transcription of VA1 and tRNA genes in vivo and in vitro was previously detected by other workers. Our results suggest that besides TFIIIC2, an additional B-box-binding protein factor may be involved in the regulation of expression of the RNA polymerase III-transcribed genes.