Barnase is a bacterial protein that has ribonuclease activity. It is lethal to the cell when expressed without its inhibitor barstar. The inhibitor binds to and occludes the ribonuclease active site, preventing barnase from damaging the cell's RNA after it has been synthesized but before it has been secreted.

Barnase catalyzes hydrolysis at diribonucleotide GpN sites. Cleavage occurs in two steps using a general acid-base mechanism: a cyclic intermediate is formed during the first transesterification step, which is then hydrolysed to release the cleaved RNA.

When synthesised by plants it causes male sterility by producing a ribonuclease enzyme (RNAse) that interferes with RNA production in specific cells of the pollen sac, thus disrupting their normal cell functioning and preventing the development of pollen which lead to male sterility.