Human endogenous retrovirus K10 encodes a functional integrase

We cloned a human endogenous retrovirus K10 DNA fragment encoding integrase and expressed it as a fusion protein with Escherichia coli maltose-binding protein, Integrase activities were measured in vitro by using a double-stranded oligonucleotide as a substrate mimicking viral long terminal repeats (LTR), The fusion protein was highly active for both terminal cleavage and strand transfer in the presence of Mn2+ on the K10 LTR substrate. It was also active on both Rous sarcoma virus and human immunodeficiency virus type 1 LTR substrates, whereas Rous sarcoma virus and human immunodeficiency virus type 1 integrases were active only on their corresponding LTR substrates, The results strongly suggest that K10 encodes a functional integrase with relaxed substrate specificity.