Catalyzes the hydrolysis of unsaturated rhamnogalacturonan disaccharide to yield unsaturated D-galacturonic acid and L-rhamnose. It cannot act on unsaturated glucuronyl hydrolase (UGL) substrates containing unsaturated D-glucuronic acid at the non-reducing terminus, although the active pockets of YesR and UGL are very similar. 2-O-(4-deoxy-beta-L-threo-hex-4-enopyranuronosyl)-alpha-L-rhamnose + H2O = 5-dehydro-4-deoxy-D-glucuronate + L-rhamnopyranose Optimum pH is 6.0. Optimum temperature is 50 degrees Celsius. It is stable below 50 degrees Celsius. Monomer. Up-regulated by growth on type I rhamnogalacturonan. Belongs to the glycosyl hydrolase 105 family.