On the advantage of being a dimer, a case study using the dimeric Serratia nuclease and the monomeric nuclease from Anabaena sp. strain PCC 7120.

The extracellular endonucleases from Serratia marcescens and Anabaena sp. are members of a family of nonspecific endonucleases. In contrast to the monomeric Anabaena nuclease, the Serratia nuclease is a dimer of two identical subunits. To find out whether the two active sites of the Serratia nuclease function independently of each other and what the advantage of being a dimer for this enzyme might be, we produced (i) dimers in which the two subunits were cross-linked, (ii) heterodimers consisting of a wild type and an inactive mutant subunit which were also cross-linked, and (iii) monomeric variants which are unable to dimerize. The monomeric H184R variant and the cross-linked S140C variant exhibit the same activity as the wild type enzyme, while the cross-linked heterodimer with one inactive subunit shows only half of the activity of the wild type enzyme, demonstrating functional independence of the two subunits of the Serratia nuclease. On the other hand at low enzyme and substrate concentrations dimeric forms of the Serratia nuclease are relatively more active than monomeric forms or the monomeric Anabaena nuclease in cleaving polynucleotides, not, however, oligonucleotides, which is correlated with the ability of dimeric forms of the Serratia nuclease to form large enzyme-substrate networks with high molecular weight DNA and to cleave polynucleotides in a processive manner. We conclude that in the natural habitat of Serratia marcescens where the supply of nutrients may become growth limiting the dimeric nuclease can fulfil its nutritive function more efficiently than a monomeric enzyme.