Purification and properties of a ribonuclease induced during the early larval development of Artemia salina

Purification and characterization of Artemia 2',3'-cyclic nucleotide 3'-phosphodiesterase

This paper describes the purification and properties of a 2',3'-cyclic nucleotide 3'-phosphodiesterase which hydrolyzes nucleoside 2',3'-cyclic monophosphates to nucleoside 2'-phosphates. The enzyme is present in encysted gastrulae of Artemia and its specific activity greatly increases during larval development. The purified enzyme has a molecular weight of around 55 000 as estimated by gel filtration, does not require metals for activity, is inhibited by Zn2+ and inactivated by Cu2+ and has a pH optimum at around neutrality. Based on the relative values of V(max)/Km, the specificity of the phosphodiesterase toward the four 2',3'-cyclic nucleotides is Guo-2',3'-P > Ado-2',3'-P > Cyd-2',3'-P > Urd-2',3'-P = 45:36:20:7. The enzyme from Artemia gastrulae is competitively inhibited by the four nucleosides 2'-phosphates (Ki values around 1 mM) while the enzyme from larvae is only inhibited by the purine nucleotides. The phosphodiesterase characterized in this work is more similar in substrate specificity to the 2',3'-cyclic nucleotide 3'-phosphodiesterase from the mammalian nervous system than to the plant enzyme. The functional relationship of this enzyme with the Artemia ribonuclease VI is discussed.

Diguanosine 5',5'''-P1,P4-tetraphosphate and other purine nucleotides inhibit endoribonuclease VI from Artemia

The activity of the endoribonuclease VI from Artemia is sensitive to several purine nucleotides. The enzyme is non-competitively inhibited by diguanosine tetraphosphate (Ki = 75 microM), a nucleotide abundant in Artemia encysted gastrulae and located in the same particulate fraction as the gastrular ribonuclease. Diguanosine triphosphate and diadenosine tetraphosphate are less efficient inhibitors (Ki congruent to 200 microM). The ribonuclease is non-competitively inhibited by 5'-AMP (Ki = 10 microM) and 5'-GMP (Ki = 50 microM) but is insensitive to the corresponding 5'-phosphates of cytosine and uridine. Other purine mononucleotides inhibit the enzyme activity less efficiently. The modulation of the enzyme activity by these nucleotides is discussed in relation with the changes in ribonuclease activity during early development of Artemia.

Purification and mode of action of a microsomal endoribonuclease from rat liver

An endoribonuclease has been purified nearly to homogeneity from rat liver microsomes, and its mode of action and general properties were studied. The enzyme had an apparent molecular weight of 58 000, as estimated by both gel filtration and SDS-polyacrylamide gel electrophoresis and produced oligonucleotides from poly(A), poly(U) and poly(C). No mononucleotide was obtained by the enzymatic hydrolysis of the above substrates. The enzyme made endonucleolytic cleavages which generated 5'-phosphate-terminated oligonucleotides. It was suggested that the existence of at least (Ado5'P)2 residues at both sides of the cleavage bond was necessary for the action of the endoribonuclease. Divalent cations (Mg2+ or Mn2+) were required for the enzymatic activity, while K+ inhibited the enzyme. Spermine stimulated the enzymatic activity in the presence of 1 mM Mg2+.

Inhibition of endoribonuclease VI from Artemia larvae by cytidine 2'-phosphate

The endoribonuclease VI from Artemia larvae is non-competitively inhibited by cytidine 2'-phosphate with a Ki ca 1 microM. Neither of the cytidine monophosphates isomers with the phosphate group in the 3' or 5' position nor the cyclic 2':3' phosphate are inhibitors at concentrations up to 100 microM. Adenosine, guanosine and uridine 2' or 3' phosphates are also ineffective in this range of concentrations. Certain polyribonucleotides are potent competitive inhibitors of the ribonuclease activity.

Developmental changes of Artemia ribonuclease

Encysted gastrulae of Artemia contain a particulate ribonuclease functionally analogous to that present in larvae. This activity is 1/200 times lower than that found in developed larvae and remains constant during embryonic development. Concomitant with the hatching of the larvae, there is a burst in enzyme activity (25-30-fold) and a change in its cellular location from the particulate fraction to the cell cytosol. The larval enzyme increases as development progresses (10-fold) and appears as two distinct peaks of activity after gel filtration. The heavy peak greatly decreases with the time of development. The results are discussed in relation to the control of the expression of this enzyme.

DNA polymerases alpha and gamma during pre-emergence and early larval development of Artemia

DNA polymerases alpha and gamma have been studied in cryptobiotic cysts and developing embryos and larvae of the brine shrimp Artemia. The two enzymes readily separate on Cibacron blue 3-GA Matrex gel. Assay requirements with activated DNA as primer-template are pH 8.0, 1 mM Mg2+, 50 mM K+ for DNA polymerase alpha and pH 8.4, 10 mM Mg2+, 80 mM K+ for DNA polymerase gamma. DNA polymerase alpha is inhibited by N-ethylmaleimide (94% and 100% at 1 mM and 10 mM respectively) and aphidicolin (96% at 60 microM). DNA polymerase gamma is also sensitive to N-ethylmaleimide (83% and 100% inhibition at 1 mM and 10 mM respectively) but is resistant to aphidicolin. 2',3'-Dideoxythymidine 5'-triphosphate (ddTTP) inhibits the gamma polymerase by 88% when in fivefold excess over dTTP whereas the alpha polymerase is unaffected by this compound. DNA polymerase alpha has a sedimentation coefficient of 7.6 S which is reduced to 6.2 S by a phenylmethylsulphonyl fluoride-sensitive proteinase. The gamma polymerase sediments at 8.3 S. No DNA polymerase beta activity could be detected. After the reinitiation of development both activities increased twofold up to 8 h (gamma polymerase) and 16 h (alpha polymerase), then declined before the onset of nuclear DNA replication after hatching. Thymidine kinase activity increased over 200-fold up to the time of replication. Analysis on Percoll density gradients of the intracellular distribution of both polymerases during development suggests that the changes in their activities may be due to migration from storage sites to replication complexes in the nuclei and mitochondria. The content of the mitochondrial DNA polymerase gamma in different batches of cysts may reflect the relative viabilities of the cysts.

Levels of the RNA polymerases during the early larval development of Artemia

Artemia nauplii contain three forms of RNA polymerase. RNA polymerases I and II have the common pattern of alpha-amanitin sensitivity of the eukaryotic enzymes, but RNA polymerase III is insensitive to high concentrations of the drug. The determination of the levels of the RNA polymerases during early larval development is affected by the increase of proteolytic activity during this stage, which produces the inactivation of RNA polymerases I and III in vitro. Protease B has been identified to be the enzyme responsible for the inactivation of RNA polymerase I. The culture conditions of Artemia larvae also affect the levels of the RNA polymerases. The maintenance of the larvae in the absence of food produces a decrease of RNA polymerases I and III several hours after hatching. In contrast, fed nauplii contain almost stable levels of the three forms of RNA polymerases during the studied period of larval development. The decrease of the levels of RNA polymerases I and III in starved nauplii could be the molecular mechanism responsible for the drop in RNA synthesis observed during the larval development in cultures of nauplii in the absence of an external source of food.

A microsomal endoribonuclease from rat liver

An endoribonuclease has been purified about 320-fold from the microsomes of rat liver. The enzyme had an apparent molecular weight of 54 000-58 000 and produced oligonucleotides, each consisting of 3-7 nucleotides from poly(A) and poly(U). No mononucleotide was obtained by the enzymatic hydrolysis of poly(A) and poly(U) under standard coditions. The relative rates of breakdown of synthetic polynucleotides by the enzyme under standard conditions were in the order poly(U) = poly(A) > poly(C). Divalent cations (Mg2+ or Mn2+) was required for the enzymatic activity, but monovalent cations (Na+, K+ or NH4+) inhibited the enzyme. The breakdown of poly(C) and poly(U) by the enzyme was inhibited by spermine, but that of poly(A) was not influenced by spermine. The enzyme was inhibited by p-chloromercuribenzoate and poly(G), but not by rat-liver ribonuclease-inhibitor and anti-RNase A serum.