Multiple Subelements within the Polyomavirus Enhancer Function Synergistically To Activate DNA Replication

Abstract

The polyomavirus origin for DNA replication comprises at least two essential, but functionally distinct, cis-acting components. One of these, the origin core, is required only for DNA replication. It includes binding sites for large T antigen and the origin of bidirectional DNA replication. The other component is required for both transcription and DNA replication and is represented by two functionally redundant regions, α and β, which are elements of the polyomavirus enhancer. Whereas either enhancer element will activate DNA replication, both enhancer elements are required to constitute a functional enhancer of transcription. To identify the sequences that make up each enhancer element, we have subjected them separately to in vitro mutagenesis and measured their capacity to activate replication in cis of the origin core in MOP-8 cells, which provide all trans-acting replicative functions including large T antigen. The results reveal that the β enhancer element is composed of three subelements, two auxiliary subelements, and a core subelement. The core subelement independently activated DNA replication, albeit poorly. The auxiliary subelements, which were inactive on their own, acted synergistically with the core subelement to increase its activity. Interestingly, dimers of the β core subelement functioned as well as the combination of a β auxiliary subelement and a core subelement, suggesting that the subelements are functionally equivalent. The α enhancer element is organized similarly; it too comprises an auxiliary subelement and a core subelement. These results lead us to suggest that the polyomavirus enhancer comprises two levels of organization; two or more enhancer elements form an enhancer, and two or more subelements make up an enhancer element. The subelements share few sequences and serve as binding sites for distinct cellular factors. It appears, therefore, that a number of different cellular proteins function cooperatively to activate polyomavirus DNA replication by a common mechanism.