Rat liver DNA polymerase-beta: thermal inactivation of RNA- and DNA-dependent activities

Partial purification and characterization of the reverse transcriptase of the simian immunodeficiency virus TYO-7 isolated from an African green monkey

The reverse transcriptase (RT) was partially purified by a newly developed procedure from the simian immunodeficiency virus TYO-7 isolated from an African green monkey (SIVagmTYO-7). The method comprised lysis of the virus with nonionic detergent followed by two centrifugations in isopycnic sucrose density gradients and one velocity sedimentation in a glycerol gradient. The enzyme exhibited a purity of 70-80% and showed an exceptional high specific activity of 135 nmol incorporation of dTMP per milligram of protein in 1 h with poly(rA).oligo(dT) as template-primer (TP). The molecular weight of the native enzyme was estimated by velocity sedimentation analysis as 120K-130K. Investigation of the RT by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the active enzyme is a heterodimer composed of a 64- and a 50-kDa subunit. The two subunits were identified to be RT specific by Western blot analysis. In activity gels, both subunits exhibited enzymatic activity, whereby the 64-kDa subunit showed the predominant activity. The RT preferred the TP poly(rA).oligo(dT) over poly(rC).oligo(dG). With poly(rCm).oligo(dG), only marginal activity was detected, and no activity was measured with poly(dA).oligo(dT). The TP specificity was influenced by the reaction temperature. The highest activity was measured around the melting temperature of the TP used. Furthermore, the enzyme activity was more thermolabile when measured with poly(rA).oligo(dT) than with poly(rC).oligo(dG). To compare the specificity of RT inhibitors, their inhibition efficiency (IE) was defined as the ratio of the 50% inhibiting concentration (ID50) obtained with the RT in viral lysates to the ID50 of purified RT.

Rapidly reversible enzyme inhibition in a temperature-sensitive mammalian cell mutant lacks thermotolerance

The temperature-sensitive (ts) Chinese hamster ovary (CHO) cell mutant tsH1 contains a thermolabile leucyl-tRNA synthetase. Upon incubation at the nonpermissive temperature of 39.5 degrees C, the enzyme became reversibly inhibited over a period of minutes, and the cells lost viability over a period of many hours. However, killing of tsH1 by acute heating at 45 degrees C was identical to that of wild-type (SC) cells. In addition, the heat-induced inhibition of protein synthesis was similar for both cell types, as measured after acute heating at 45 degrees C. Furthermore, both killing and inhibition of protein synthesis showed thermotolerance in both cell types. In contrast to the effects at 45 degrees C, at 39.5 degrees C, neither the inhibition of leucyl-tRNA synthetase activity nor the killing of tsH1 expressed thermotolerance. Also, treatment of tsH1 at 39.5 degrees C did not induce thermotolerance to killing at 45 degrees C. The inhibition of leucyl-tRNA synthetase activity in tsH1 at 39.5 degrees C was further distinguished from the 45 degrees C-induced inhibition of protein synthesis in SC cells by a much more rapid reversal of the inhibition of leucyl-tRNA synthetase activity. Also, the rate of reversal of the inhibition of protein synthesis by 45 degrees C in SC cells was decreased by increased heat dose. Such was not true for the 39.5 degrees C inhibition of leucyl-tRNA synthetase activity in tsH1. The data indicate that there exist two distinct types of thermal inhibition--one slowly reversible type which was observed during and after heating at 45 degrees C and both induced and expressed thermotolerance, and a second, rapidly reversible type, which was evident only during heating of tsH1 at 39.5 degrees C and neither induced nor expressed thermotolerance.