On the advantage of being a dimer, a case study using the dimeric Serratianuclease and the monomeric nuclease from Anabaena sp. strain PCC 71209

The extracellular endonucleases from Serratia marcescens and Anabaena sp. a re members of a family of nonspecific endonucleases. In contrast to the mon omeric Anabaena nuclease, the Serratia nuclease is a dimer of two identical subunits, To find out whether the two active sites of the Serratia nucleas e function independently of each other and what the advantage of being a di mer for this enzyme might be, we produced (i) dimers in which the two subun its were cross-linked, (ii) heterodimers consisting of a wild type and an i nactive mutant subunit which were also cross-linked, and (iii) monomeric va riants which are unable to dimerize, The monomeric H184R variant and the cr oss-linked S140C variant exhibit the same activity as the wild type enzyme, while the cross-linked heterodimer with one inactive subunit shows only ha lf of the activity of the wild type enzyme, demonstrating functional indepe ndence of the two subunits of the Serratia nuclease, On the other hand at l ow enzyme and substrate concentrations dimeric forms of the Serratia nuclea se are relatively more active than monomeric forms or the monomeric Anabaen a nuclease in cleaving polynucleotides, not, however, oligonucleotides, whi ch 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 be come growth limiting the dimeric nuclease can fulfil its nutritive function more efficiently than a monomeric enzyme.