Specific protein phosphorylation in interferon-treated uninfected and virus-infected mouse L929 cells: enhancement by double-stranded RNA

The alpha subunit of eucaryotic initiation factor 2 is phosphorylated in mengovirus-infected mouse L cells

Infection of mouse L cells with mengovirus resulted in the activation of a protein kinase (PK) that selectively phosphorylated the small, 38,000-molecular-weight alpha subunit of eucaryotic initiation factor 2 (eIF-2) in vitro. The mengovirus-activated kinase was detected in vitro approximately 3 h after virus adsorption. The ratio of phosphorylated to unphosphorylated eIF-2 also increased in vivo between 3 and 7 h after adsorption. The virus-activated kinase fractionated with the ribosomal pellet and had a high affinity for DEAE-cellulose and Mono Q ion-exchange columns. Gel electrophoresis of the kinase activity eluting from the Mono Q column and silver staining of the gel revealed only one protein band with a molecular mass of 70 kilodaltons. The optimal assay conditions for the mengovirus-activated kinase paralleled those of the double-stranded RNA-activated PK (dsRNA-PK). Lysates from infected cells contained elements capable of activating partially purified dsRNA-PK. These elements were identified as double-stranded RNA by their sensitivity to double-stranded RNase. The phosphorylation of the alpha subunit of eIF-2 coincided with the synthesis of dsRNA in infected cells, suggesting that the mengovirus-activated kinase is the dsRNA-PK. The phosphorylation of the alpha subunit of eIF-2 correlated with the global inhibition of protein synthesis that occurs at late times after infection.

Mechanism of interferon action. Activation of the human P1/eIF-2 alpha protein kinase by individual reovirus s-class mRNAs: s1 mRNA is a potent activator relative to s4 mRNA

The ability of pure viral and cellular single-strand (ss) RNAs to activate the interferon-induced, double-stranded (ds) RNA-dependent P1/eIF-2 protein kinase purified from human amnion U cells was examined. In addition to the well-established activation of P1 kinase autophosphorylation in vitro by reovirus genome dsRNA, the P1 kinase was also efficiently activated by certain reovirus ssRNAs. The reovirus s1 mRNA was a potent activator of the kinase. By contrast, the reovirus s4 mRNA was a poor activator of the kinase. Likewise, adenovirus VAI RNA, transfer RNA, 5 S ribosomal RNA, and rabbit globin mRNA were not activators or were very poor activators of the purified P1/eIF-2 protein kinase. Analysis of hybrid ssRNAs produced between the reovirus s1 and s4 mRNAs revealed that both the 5' and the 3' portions of the s1 mRNA possessed nucleotide sequences capable of mediating kinase activation. Subsequent deletion analysis of the 5' portion of the s1 mRNA identified a 161-nucleotide region located between positions 416 and 576 which was sufficient for P1 kinase activation. Treatment of reovirus s1 mRNA transcripts with either ssRNA- or dsRNA-specific ribonucleases, but not with heat, destroyed the ability of s1 mRNA transcripts to activate the kinase. These results suggest that P1 kinase autophosphorylation in vitro may be selectively activated by individual ssRNAs in a differential manner, and that a secondary or higher-ordered ssRNA structure(s) may be important in mediating the activation.

A kinase able to phosphorylate exogenous protein synthesis initiation factor eIF-2 alpha is present in lysates of mengovirus-infected L cells

Infection of mouse L929 cells by mengovirus resulted in the expression of a kinase activity that selectively phosphorylated the small, 38,000-molecular-weight subunit of eucaryotic initiation factor 2 and histone H2. This kinase activity was independent of host cell RNA synthesis and was located in the postribosomal supernatant (S-100 fraction) early after infection (up to 3 h). At later times after infection (5 h), kinase activity was also associated with the polysome fraction. The kinase present in the S-100 fraction bound strongly to DEAE-cellulose, its peak activity eluting at 0.5 M KCl. Kinase activity was independent of the presence of exogenous double-stranded RNA, and KCl at concentrations greater than 0.1 M inhibited eucaryotic initiation factor 2 phosphorylation. The 67,000-molecular-weight phosphoprotein activated in interferon-treated cells by double-stranded RNA was not detected by standard phosphorylation assays in lysates from mengovirus-infected cells. Labeling of this protein in vivo during 5 h of infection was also not detected. The DEAE-cellulose-purified mengovirus kinase inhibited protein synthesis in reticulocyte lysates, and the inhibition was not reversible by high concentrations of poly(I).poly(C).

Double-stranded RNA-dependent protein kinase and 2-5A system are both activated in interferon-treated, encephalomyocarditis virus-infected HeLa cells

Activation of the interferon-inducible, double-stranded RNA-dependent protein kinase was monitored in monolayer cultures of control and interferon-treated HeLa cells infected with encephalomyocarditis virus. The extent of phosphorylation in the intact cell of the alpha-subunit of eucaryotic protein synthesis initiation factor eIF2 by the kinase was determined for the first time in this type of system, using a two-dimensional immunoblot technique. Virus protein synthesis and the kinetics of activation of the ppp(A2'p)nA (n greater than or equal to 2) system were analyzed in parallel. Enhanced phosphorylation of eIF2-alpha was obvious at 9 h and increased by 12 h postinfection. ppp(A2'p)nA and ppp(A2'p)nA-mediated rRNA cleavage were observed from 6 h. No viral protein synthesis was detected in cells in which a general inhibition of protein synthesis developed with time. It can be concluded that both the kinase and ppp(A2'p)nA system are active in interferon-treated, encephalomyocarditis virus-infected HeLa cells.

Receptors for human gamma interferon: binding and crosslinking of 125I-labeled recombinant human gamma interferon to receptors on WISH cells

Purified recombinant human gamma interferon (HuIFN-gamma), labeled with 125I (125I-HuIFN-gamma), was used to study receptors for HuIFN-gamma on human WISH cells. 125I-HuIFN-gamma was bound to WISH cells, and this binding was displaced by unlabeled HuIFN-gamma but not by unlabeled recombinant HuIFN-alpha 2 or [Ser17]HuIFN-beta (HuIFN-beta with serine substituted for cysteine at position 17), indicating the presence of specific binding sites for HuIFN-gamma. The cell-bound 125I-HuIFN-gamma was crosslinked with disuccinimidyl suberate or ethylene glycol bis(succinimidyl succinate), which yielded a complex of Mr approximately 105,000 +/- 5000 as analyzed by NaDodSO4/PAGE. The formation of this complex was prevented by preincubation of cells with unlabeled HuIFN-gamma but not with HuIFN-alpha 2 or [Ser17]HuIFN-beta, indicating that HuIFN-gamma binds to a specific receptor molecule and that HuIFN-alpha 2 or HuIFN-beta do not interact with this receptor. Experiments were carried out with 125I-labeled recombinant [Ser17]HuIFN-beta (125I-[Ser17]HuIFN-beta) to verify this conclusion. Binding and crosslinking of 125I-[Ser17]HuIFN-beta to human WISH cells and Daudi cells yielded a complex of Mr approximately 150,000 similar to that obtained with 125I-HuIFN-alpha 2 as described earlier. The formation of this Mr 150,000 complex with 125I-[Ser17]HuIFN-beta was displaced by unlabeled [Ser17]HuIFN-beta and by HuIFN-alpha 2 but not by HuIFN-gamma, indicating that [Ser17]HuIFN-beta binds to the same receptor as does HuIFN-alpha 2, identified earlier, and that HuIFN-gamma does not compete with 125I-[Ser17]HuIFN-beta for this receptor. We conclude that HuIFN-gamma interacts with specific receptors that are distinctly different from the receptors recognized by HuIFN-alpha and HuIFN-beta.

Interferon-mediated protein kinase activity in different fractions of mouse L-929 cells

The interferon (IFN)-mediated protein kinase activity in extracts from mouse L-929 cells is manifested by the phosphorylation of an endogenous 67 kD molecular weight (mw) protein in the presence of double-stranded (ds) RNA. This protein kinase activity can also be assayed after partial purification on poly(I) X poly(C)-Sepharose under phosphatase-free conditions. By the use of this latter technique, here we investigated the distribution of the protein kinase activity in different cellular compartments. Most of the protein kinase activity is found in the post-ribosomal supernatant (S100) fraction, while a small portion of it is associated with the ribosomal salt wash (RSW: 0.5 M KCl eluate of ribosomal pellet) and nuclear fractions. These results are in contrast to several observations in the literature in which the protein kinase activity is thought to be associated with the ribosomal pellet. This controversy results from the conditions used for assay of the protein kinase activity. In fact, when the kinase is assayed in crude extracts supplemented with dsRNA, very little kinase activity is detectable in the S100 fraction compared to the RSW fraction. The S100 fraction contains a high level of phosphatase(s) activity which interferes with the protein kinase assay and might account for the misinterpretation observed in the literature. Some recent results have implicated a correlation between the dsRNA-dependent protein kinase responsible for the phosphorylation of the 67 kD protein and a polyamine-dependent protein kinase which phosphorylates a similar molecular weight protein, subunit of ornithine decarboxylase (Orn Dcase). Here, we show that Orn Dcase does not bind to poly(I) X poly(C)-Sepharose and polyamines do not substitute the requirement of dsRNA for the phosphorylation of the 67 kD protein.

Effect of interferon treatment on blockade of protein synthesis induced by poliovirus infection

Treatment of HeLa cells with lymphoblastoid interferon leads to a drastic inhibition of infective poliovirus. Even relatively high concentrations of human lymphoblastoid interferon HuIFN-alpha (Ly) (400 IU/ml) do not prevent destruction of the cell monolayer after most of the cells have been infected with poliovirus. Analysis of macromolecular synthesis in a single step growth cycle of poliovirus in interferon-treated cells detected no viral protein synthesis. In spite of this inhibition of viral translation, the shut-off of host protein synthesis in interferon-treated cells is apparent when they are infected both at low and high multiplicities. Although viral RNA synthesis is inhibited considerably in cells treated with interferon, a certain amount is detected, suggesting that some viral replication takes place. Analysis of membrane permeability after poliovirus infection shows a leakage to 86Rb+ ions and modification of membrane permeability to the translation inhibitor hygromycin B at the moment when the bulk of virus protein synthesis occurs. These changes are delayed and even prevented if cells are pretreated with interferon. A situation is described in which host protein synthesis is shut-down with no major changes in membrane permeability, as studied by the two tests mentioned above. Prevention of viral gene expression by inactivation with ultraviolet light of the input virus or by treatment with cycloheximide blocks the shut-off of protein synthesis. This does not occur in the presence of 3 mM guanidine. These observations are in agreement with the idea that some poliovirus protein synthesis takes place in interferon-treated cells and this early gene expression is necessary to block cellular protein synthesis.

Cellular RNA is not degraded in interferon-treated HeLa cells after poliovirus infection

A drastic inhibition of protein synthesis occurs in HeLa cells treated with human lymphoblastoid interferon and infected with poliovirus. At the time when this inhibition has been established no degradation of 32P-labelled ribosomal RNA can be detected. Isolation of the mRNAs from poliovirus-infected cells plus or minus interferon treatment, followed by translation in a reticulocyte lysate indicates that cellular mRNAs remain active. These results suggest that gross degradation of cellular RNA does not occur in interferon-treated poliovirus-infected HeLa cells and that a non-specific nuclease induced by 2'-5' A is not responsible for the inhibition of protein synthesis observed.

Differential antiviral effects of interferon in three murine cell lines

Infectious leukemia virus production by two chronically infected NIH/MOL lines was strongly inhibited by interferon treatment of the cells. The corresponding degree of inhibition in JLSV-11 cells was much lower. Multiplication of encephalomyocarditis virus in all three cell lines was barely affected by interferon treatment. Replication of vesicular stomatitis virus, on the other hand, was highly sensitive to interferon in the JLSV-11 line and in one NIH/MOL line but was practically insensitive in the other NIH/MOL line. Anticellular actions of interferon were more pronounced in the JLSV-11 line than in the others. In response to interferon treatment, 2',5'-oligoadenylate synthetase activity was induced to a high level in JLSV-11 cells and to lower levels in the NIH/MOL lines. We failed to detect any 2',5'-oligoadenylate-dependent endonuclease activity in extracts of these cells. Double-stranded RNA-dependent protein kinase activity was present in extracts of interferon-treated NIH/MOL cells, but it was barely detectable in extracts of interferon-treated JLSV-11 cells. The above studies demonstrated that interferon could differentially affect the replication of three different viruses in three different cell lines, including two seemingly identical NIH/MOL lines, and that certain tentative conclusions can be drawn regarding the roles of different interferon-inducible enzyme markers in the different antiviral actions of interferons.

Double-stranded RNA-dependent phosphorylation of protein P1 and eukaryotic initiation factor 2 alpha does not correlate with protein synthesis inhibition in a cell-free system from interferon-treated mouse L cells

The double-stranded RNAs (I)n X (C)n and (A)n X (dUfl)n (dUfl is 2'-fluoro-2'-deoxyuridylic acid) have been compared as inhibitors of translation in cell-free systems from interferon-treated mouse L cells and from rabbit reticulocytes. In the interferon-treated mouse L-cell system, both double-stranded RNAs stimulated kinase activity, leading to phosphorylation of protein P1 and eukaryotic initiation factor 2 alpha (eIF-2 alpha), but only (1)n X (C)n activated the (2'-5')-oligoadenylate synthetase. Moreover, in this system, (I)n X (C)n, but not (A)n X (dUfl)n, inhibited translation. Both (A)n X (dUfl)n and (I)n X (C)n also activated the rabbit reticulocyte kinase to phosphorylate protein P1 and eIF-2 alpha, but, in contrast to mouse L-cell systems, both (A)n X (dUfl)n and (I)n X (C)n were potent inhibitors of translation in reticulocyte lysates. These results indicate that protein P1 and eIF-2 alpha phosphorylation are not sufficient to cause inhibition of protein synthesis in interferon-treated mouse L-cell extracts. They further suggest that protein synthesis inhibition by (I)n X (C)n in extracts of interferon-treated L cells correlates better with activation of (2'-5')-oligoadenylate synthetase than with activation of the protein P1 and eIF-2 alpha kinase.

Regulation of the antiviral and anticellular activities of interferon by exogenous double-stranded RNA

The effects of double-stranded RNA (dsRNA) on interferon (IFN)-induced antiviral and anticellular activities was investigated by introducing poly(I)-poly(C) into mouse L-cells. Coprecipitation of dsRNA with calcium phosphate enabled its efficient penetration into cells in culture. Rate of cellular protein synthesis was inhibited by dsRNA only in cultures pretreated with IFN. Moreover, the anticellular effect of IFN, as measured by the inhibition of cell DNA synthesis, was also enhanced by dsRNA. The kinetics of dsRNA-mediated inhibition of protein synthesis were relatively slow as compared with the inhibitory effect of 2'-5' oligoadenylic acid (2'5'A), which was also introduced into cells by the calcium phosphate coprecipitation technique. To analyze the effects of dsRNA on the antiviral state induced by IFN, vesicular stomatitis virus (VSV) and encephalomyocarditis virus (EMC), replications were followed by measuring viral-specific RNA synthesis in the cell. Introduction of dsRNA after the infection had no effect on VSV and EMC replication in control cells, and it enhanced, to a small extent, the antiviral state of cells pretreated with IFN. In contrast, introduction of 2'5'A into virus-infected cells inhibited VSV and EMC replications regardless of IFN pretreatment. This work demonstrated that the role of dsRNA in regulating the antiviral and anticellular activities of IFN could be studied by introducing exogenous dsRNA into cells in culture by the calcium phosphate coprecipitation technique.

Interferon action in human fibroblasts: induction of 2', 5'-oligoadenylate synthetase in the absence of detectable protein kinase activity

In several strains of human fibroblasts, treatment with human alpha- or beta-interferon (IFN) resulted in an induction of 2', 5'-oligoadenylate synthetase but no protein kinase activity. The antiviral and anticellular effects of IFN are elicited in these cells in the absence of detectable IFN-induced protein kinase activity.

Mechanism of interferon action: eIF-2 alpha phosphatase in interferon-treated mouse fibroblasts is double-stranded RNA independent

The effect of double-stranded RNA on the dephosphorylation of purified, 32P-labeled eIF-2 was examined in cell-free extracts prepared from interferon-treated mouse L929 cells. Dephosphorylation of the alpha subunit of eIF-2 occurred at comparable rates in the presence and in the absence of reovirus dsRNA. By contrast, the beta subunit of eIF-2 was not dephosphorylated to any significant extent in either the presence or the absence of dsRNA. These results indicate that the enhanced phosphorylation of eIF-2 alpha observed in IFN-treated systems in the presence of double-stranded RNA (dsRNA) is indeed caused by an activation of a protein kinase rather than to an inhibition of a phosphoprotein phosphatase by the dsRNA.

Lack of systematic correlation between the interferon mediated antiviral state and the levels of 2-5A synthetase and protein kinase in three different types of murine cells

The levels of two dsRNA-dependent enzyme activities, the pppA(2'p5'A)n synthetase (2-5A synthetase) and protein kinase were investigated in control and interferon-treated murine cells: L-929, K/Balb and NIH/3T3. Treatment of these cells with interferon resulted both in the establishment of the antiviral response and the development of anticellular effects. This latter was observed 3 days after treatment with interferon. The levels of 2-5A synthetase and protein kinase in control and interferon-treated cells seemed to vary from one cell type to the other. In L-929 cells, both the 2-5A synthetase and protein kinase were induced by interferon as has been shown previously. On the other hand, treatment of NIH/3T3 cells with interferon resulted in the induction of 2-5A synthetase in the absence of any enhanced protein kinase activity. This lack of protein kinase in interferon-treated NIH/3T3 cells was not due to the presence of high levels of protein phosphatases. A third type of mouse cells, K/Balb cells, contained very high levels of 2-5A synthetase in the absence of any apparent resistance to virus infection. On treatment with interferon the level of 2-5A synthetase in K/Balb cells remained constant while the protein kinase activity was enhanced by several fold. Both control and interferon-treated K/Balb cells showed similar sensitivity to the action of exogenous 2-5A thus suggesting that the 2-5A system (the 2-5A dependent nuclease and the phosphodiesterase which degrades 2-5A) was not altered on treatment with interferon. The significance of these results in relation to the mechanism of action of interferon is discussed.

Double-stranded RNA and the enzymology of interferon action

Extracts from interferon-treated, not virus-infected Ehrlich ascites tumor cells differ in various biochemical characteristics from extracts of control cells. We studied three enzymes whose level is enhanced in cells upon treatment with IF and which are causing some of the differences. (2'-5')(A)n synthetase, an enzyme converting ATP into a series of (2'-5') linked oligoadenylates ((2'-5')(An)) in the presence of dsRNA was purified to homogeneity and characterized. The second enzyme, RNase L, a latent endonuclease, which can be activated by (2'-5')(A)n to cleave single-stranded RNAs, was purified several hundredfold. The activation of this enzyme is reversible and is lost upon removal of (2'-5')(A)n. The activation is not accompanied by a large change in shape of conformation of the enzyme. The third enzyme is a protein kinase which if activated by dsRNA can phosphorylate the peptide chain initiation factor eIF-2 and a protein designated P1 of 67,000 daltons. This enzyme was purified several thousandfold. The most highly purified preparation consists of three proteins with P1 as the most abundant component.

Abnormal behavior of interferon-induced enzymatic activities in an interferon-resistant cell line

Interferon induces two double-stranded RNA-dependent enzymatic activities: an oligoisoadenylate synthetase that converts ATP to ppp(A2'p)n5'A, and a protein phosphokinase. We have explored the level and inducibility of these two enzymes in a human cell line (HEC-1) totally insensitive to both the antiviral and the anticellular actions of interferon. The activities of both enzymes are high in untreated cells and only minor changes occur after treatment with interferon, even at high concentrations. Interferon-treated HEC-1 cells do not contain an inhibitor of the oligoisoadenylate synthetase activity. The products of this HEC-1 oligoisoadenylate synthetase consist mainly of dimers, trimers, and tetramers as found in other cell lines after interferon treatment. The synthetase level is unaffected by treating the cells with anti-interferon antiserum, indicating that the results cannot be explained by a spontaneous low production of interferon by these cells. Furthermore, virus multiplication is not inhibited, even after treatment with interferon. These observations suggest that either the two enzymatic activities do not suffice for the establishment of an antiviral state in vivo or that a regulatory control mechanism, lost in these cells and common for both enzymes, is required for the expression of the antiviral action of interferon. This might explain both the constitutivity of the two enzymes and the interferon resistance observed.

Interferon-induced proteins in human fibroblasts and development of the antiviral state

Treatment of human fibroblasts with interferon induces the synthesis of several proteins, as detected by incorporation of [35S]methionine followed by analysis of cell extracts by polyacrylamide gel electrophoresis. The induction of these proteins had features in common with the development of the antiviral effect of interferon, such as (i) sensitivity to actinomycin D and cycloheximide when these compounds were added together with interferon, (ii) insensitivity to actinomycin D if the actinomycin D was added 2 h after the addition of interferon, (iii) similar dependence on interferon concentration, and (iv) species specificity for interferon. When interferon treatment was given in the presence of cycloheximide and actinomycin D was added before the removal of cycloheximide, all four proteins were induced, thus suggesting that their inductions are coordinated. Labeling for 2-h periods at varying time intervals after the addition of interferon revealed that the synthesis of these proteins was induced within a few hours, peaked at different time intervals, and was soon followed by a marked decline, suggesting that the mRNA's for these proteins have short half-lives. Moreover, this decline occurred despite the fact that the cells were continuously exposed to interferon, and there was no measurable loss of interferon activity in the medium. This suggests that the induction of these proteins is transient and is apparently subject to further control.

Induction of interferon in HeLa cells of a protein kinase activated by double-stranded RNA

Treatment of HeLa cells with human fibroblast interferon results in increased levels of latent protein kinase activity that can be activated by double-stranded RNA (dsRNA). The protein kinase activity in extracts of interferon-treated cells is assayed by two methods: (a) inhibition of protein synthesis in rabbit reticulocyte lysates and (b) phosphorylation of two polypeptides of Mr 72000 (P1) and 38000 (the eIF-2 alpha subunit of initiation factor 2). When extracts of interferon-treated cells are fractionated by centrifugation at 150000 x g, the protein kinase activity is found in the pellet fraction. The kinase is maximally activated by 0.1 micrograms/ml poly(I) . poly(C). An increase in protein kinase activity is detected after 8 h of treatment with 100 units interferon/ml or after a 17-h treatment with 12.5 units/ml or greater interferon concentrations. Therefore, the kinase activity increases as a function of both time of treatment and interferon concentration. Addition of actinomycin D to cells during interferon treatment prevents this increase. The protein kinase activity decreases gradually over three days when interferon-treated cells are subsequently grown in the absence of interferon.

Interferon action: induction of specific proteins in mouse and human cells by homologous interferons

Treatment of mouse (Ehrlich ascites tumor and L929) and human (FS4, GM258, etc.) cells with homologous interferons results in the induction of several proteins. Extracts obtained from cells labeled with [35S]methionine in the absence or presence of interferon were fractionated on poly(I) . poly(C)-agarose columns. The proteins retained on the columns revealed, upon sodium dodecyl sulfate/polyacrylamide gel electrophoresis, three protein bands in mouse cells (Mr 120,000; 80,000; and 67,000) and two in human cells (Mr 120,000 and 80,000) which were detected in the extracts of interferon-treated but not of untreated cells. These proteins were retained on double-stranded RNA [poly(I) . poly(C)-agarose] columns but very poorly, if at all, on single-stranded RNA [poly(I)- or poly(C)-agarose] columns, suggesting that they have an affinity for double-stranded RNA. In addition, interferon treatment of human fibroblasts greatly increased the labeling of three other protein bands (Mr 88,000; 67,000; and 56,000) which were detected in whole extracts but were not appreciably retained on poly(I) . poly(C)-agarose columns. The appearance of the induced proteins was blocked by actinomycin D if added together with interferon, indicating that transcription of certain genetic information is required. The possible correlation between the induced proteins described here and the elevated levels of certain enzymes in interferon-treated cells (a protein kinase and 2'-5'-oligoadenylate synthetase) is at present unclear.

Kinetics of the induction of three translation-regulatory enzymes by interferon

Three enzymes that cause inhibition of mRNA translation, eukaryotic initiation factor 2 protein kinase PK-i, oligoisoadenylate synthetase E, and phosphodiesterase 2'-PDi, have been recently isolated from interferon-treated cells. We show that the rise in these three enzyme activities may be used to study the response of uninfected cells to interferon. For each enzyme, a specific microassay that can be carried out on extracts from 2-5 x 10(4) monolayer cells from mouse, monkey, or man was developed. With these assays, the kinetics of induction of the three enzymes in mouse L cells are compared. The dose dependence for protein kinase PK-i induction is shown to be similar to that for the development of the antiviral state. Actinomycin D and anti-interferon serum block enzyme induction if added to the cells early after interferon treatment. The quantitative measurements of the intracellular level of these enzymes provide a new and convenient model to study the cell's response to interferon.