NUCLEOTIDES I35 



a few oligo-nucleotides. Whilst the properties of the nu- 

 cleases of mammals and yeast have been studied in some 

 detail, the corresponding bacterial enzymes have received 

 comparatively little attention. Streptococcus haemolyticus 

 (Group A) secretes both types of nuclease into the 

 medium [34, 8], and ribonuclease is particularly active 

 during the initial stages of bacterial autolysis [cf. 23]. At 

 least as far as ribonuclease is concerned, depolymerization 

 is due to hydrolysis of the bond linking C'-5 of one nucleo- 

 tide to the phosphate group attached to C'-3 (or C'-2) of 

 an adjacent pyrimidine nucleotide. Analogous linkages be- 

 tween a pyrimidine and a purine nucleotide or between 

 two purine nucleotides are stable to ribonuclease, and the 

 enzyme may therefore be regarded as being a highly specific 

 phosphodiesterase [38]. Nuclease activity can be determined 

 by using a solution of the appropriate nucleic acid and fol- 

 lowing (i) changes in viscosity, (ii) the appearance of acid- 

 soluble phosphorus compounds, (iii) the appearance of 

 acidic groups or (iv) the decrease in the absorption of ultra- 

 violet light of wavelength 300 m/<. 



After nucleotides have been dephosphorylated by appar- 

 ently non-specific phosphatases, the resultant nucleosides 

 may be attacked in one of three ways [28]. One type of 

 nucleosidase catalyses their phosphorolytic decomposition 

 to the free base and a pentose phosphate by transferring the 

 sugar moiety to inorganic orthophosphate (reaction a). The 

 phosphorolytic nucleosidases of Esch. colt exhibit specificity 

 with regard to the nitrogenous base in that they attack either 

 purine compounds or pyrimidine compounds, but not both, 

 and yet no specificity is shown towards the sugar part of the 

 substrates. Lactobacillus helveticus [33] and Esch. coli [28] 

 possess another type of transferase, specific for deoxyribo- 

 sides and utilizing not inorganic phosphate but a nitrogen- 

 ous base as the acceptor (reaction b). 



(a) uracil-riboside+H3P04 ^^ uracil+ribose-i -phosphate 

 (6) uracil— deoxyriboside+thymine ^^ 



th>Tnin? —deoxyriboside + uracil 



The hydrolytic decomposition of nucleosides to the free 



