= Ribonuclease H
{c}
<Ribonuclease H> cleaves the RNA component of an <RNA-DNA hybrid>, a paired structure containing one <RNA> strand and one <DNA> strand. Its substrate recognition therefore concerns both chemical composition and pairing, rather than merely the presence of RNA anywhere in a sample.<Ribonuclease H in IUBMB Enzyme Nomenclature>[EC 3.1.26.4, endonucleolytic cleavage and hybrid substrate]{ref}
A 1973 calf-thymus study purified the enzyme more than three thousandfold and tested several substrates. It cleaved an artificial DNA/RNA polymer pair and a hybrid made with phage DNA, while the tested <ribosomal RNA> was not attacked. Activity also required a suitable metal ion.<Purification and properties of ribonuclease H of calf thymus by J. G. Stavrianopoulos and others>[Abstract, purification, substrates and metal-dependent activation]{ref}
The comparison illustrates a specificity test. Detecting cleavage in one preparation would show activity, while testing alternative substrates helps establish what the activity recognizes. Conversely, failure to cleave a hybrid under conditions lacking an activating ion would not establish that the hybrid is intrinsically resistant. <Protein purification>, substrate controls and assay conditions jointly support the molecular interpretation; the behavior of one preparation should not be assumed to specify every member of the broader enzyme family.
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