Xrep, a plasmid-stimulating X chromosomal sequence bearing similarities to the BK virus replication origin and viral enhancers

The human X chromosome-linked fragment, "Xrep," was sequenced because it exerts a positive effect on plasmid growth in both E. coli and Saccharomyces cerevisiae. The sequence revealed three features similar to the human BK virus replication origin: Xrep has a true palindrome, CCTCC(T)3CCTCC, which is similar to "true" palindrome-like sequences found at the replication origins of polyoma [CCTC(T/C)10CTCC], BK [CCTC(A/G)8CCTCC] and SV40 [CCTCC(A)6GCCTCC] viruses. Twenty nucleotides away from the true palindrome, Xrep has the sequence GAATCCTATTCACTTTT while BK virus, the human analogue of SV40, has GAAATCCCTATTCTTTT in exactly the same position relative to the true palindrome. These two 17-mers differ only in the positions of two nucleotides comparing Xrep and BK virus. Also similar to the replication origins of DNA viruses, Xrep appears to have a cluster of enhancers adjacent to the origin-like sequences. Potent enhancer-like activity was detected in pSV1 X CAT/Xrep constructs. Xrep may originate from an endogenous virus, or from an X chromosomal replication origin.