ABSTRACT
Enhancer or activator sequences are transcriptional controlling elements that exhibit some remarkable features. Alterations within a region of the polyoma virus genome which contains the enhancer function enables the mutated virus to grow in nondifferentiated teratocarcinoma stem cells. Alternatively, specific secondary or tertiary structures could be required for the interaction of the enhancer with cellular factors; this alteration of nucleotides essential for the formation of a particular structure may eliminate both the structure and its function. In order to confirm the physical structure and to determine the individual point mutations, the nucleotide sequence of several mutants in the critical region was determined. The comparison revealed that each enhancer element harbors certain nucleotides also characteristic of the simian virus 40 core elements. Although activator elements often seem to reside within repeated sequences, very little information is available concerning the critical sequences necessary for enhancer function.
