Partial purification and properties of A K+- and Mg2+-dependent phosphodiesterase from Salmonella typhimurium

A phosphodiesterase (orthophosphoric diester phosphohydrolase, EC 3.1.4.1) which has been partially purified from the lysate of P22(C1)-infected spheroplasts of Salmonella typhimurium is dependent on either K+ or Mg2+ for its activity and displays maximal activity in the presence of both. The enzyme is practically free from interfering activities and specifically hydrolyses single-stranded polyribonucleotides leading to the production of 5′-nucleotides. The homopolymers are hydrolysed in the following sequence: poly A > poly U > poly C . Poly C and tRNA are poorly hydrolysed, and poly I is not hydrolysed at all. The enzyme is most active at neutral or slightly alkaline pH. It is thermolabile but could be protected against heat denaturation by native and denatured DNA's as well as by poly A. During the hydrolysis of homopolymers no oligonucleotides could be detected as intermediate products, which indicates the exonucleolytic nature of the enzyme. Though there are some differences in properties, the behaviour of this phosphodiesterase is similar to that of ribonuclease II of Escherichia coli.