Recombinant Methanocaldococcus jannaschii Uncharacterized protein MJ1321 (MJ1321), partial

An endodeoxyribonuclease that may play a role in defense against invading genetic elements. Uses short DNA sequences as guides to bind complementary target strands, resulting in slicing of the target DNA. Also has guide-independent activity on target DNA. Probably a first round of guide-independent activity on an invading plasmid or virus would generate guide DNAs for subsequent, more efficient, guide-dependent degradation of invading nucleic acids. Endonucleolytically cleaves DNA in short dsDNA (the guide DNA indicates where to cleave on the target DNA). Efficient guide-dependent target DNA cleavage requires a minimal length of 15 bp and is maximal at 19 bp. Prefers guide DNA with 5'-phosphorylated purines and 3'-pyrimidines; changing these bases alters the cleavage activity and patterns. Has no activity on substrate with a mismatch at positions 10 and 11, ssDNA or RNA, nor on DNA-RNA hybrids. Digests longer (750 bp) dsDNA as well as circular plasmid and naked genomic DNA, but not chromatin, in a guide DNA-independent manner. Addition of endogenous histone A3 protects DNA from cleavage, while cleavage is insensitive to methylation. When plasmid encoding active or mutated protein (Ala-541) is transformed into Sulfolobus acidocaldarius about 25-fold fewer transformants are found with active protein; reduced levels of plasmid are found in wild-type transformed cells. While S.acidocaldarius grows at a similar temperature to M.jannaschii (70 to 80 degrees Celsius) it has very different histone-like proteins, which presumably do not protect against MjAgo. Binds ssDNA, dsDNA and DNA-RNA hybrids; binding is most efficient with dsDNA.