Terminally directed hydrolysis of duplex ribonucleic acid catalyzed by a species of the BAL 31 nuclease from Alteromonas espejiana

The extracellular nuclease activities of Alteromonas espejiana sp. BAL 31 are mediated by at least two distinct protein species that differ in molecular weights and catalytic properties. The two species that have been purified to homogeneity and characterized, the "fast" (F) and "slow" (S) enzymes, both possess an exonuclease activity that shortens both strands of duplex DNA, with the F nuclease displaying a much greater (approximately 27-fold) turnover number for this degradation than the S species. In the present thesis, it is shown that the F species also mediates the terminally directed hydrolysis of a linear duplex RNA, gradually shortening molecules of this substrate through a mechanism that results in the removal of nucleotides from both the 3' and the 5' ends. This degradation proceeds with very infreguent introduction of scissions away from the termini as demonstrated by gel electrophoretic examination of the products of partial degradation, both in duplex form and after denaturation by reaction with CH3HgOH, and by electron microscopic characterization of duplex partially degraded molecules. Tha apparent Michaelis constant and turnover number have been determined. At eguimolar enzyme concentrations in the limit of high substrate concentration, the F nuclease will degrade duplex RNA at a rate 0.019 [plus-minus] 0.004 times that for a duplex DNA of comparable guanine + cytosine content. The S species, by contrast, showed very little activity against the duplex RNA substrate in the photometric assay.