Determination of specific radioactivity of UTP in HeLa cells by an enzymatic method and its alteration under various growth conditions

Uridine kinase molecular species and uridine uptake in some variants of rat hepatomas

In a family of clonal lines derived from the Reuber H 35 rat hepatoma, four electrophoretically distinct molecular forms of uridine kinase (UK I, II, III, and IV) have been characterized. They are the same as those found in foetal rat liver. Different UK profiles occur in these cell lines, and no strict correlation could be established between the state of differentiation of the cells and the form of UK expressed. A clone of somatic hybrid cells between line p4 (form 1 only) and Fu5-5 (forms II, III, and IV) that does not express form I indicates that p4 cells may lack a factor controlling the polymerization of form I. This variety of clonal cell lines was used to study the uptake and phosphorylation of labeled uridine. The results suggest a relationship between the UK form present and the rate uridine phosphorylation by the intact cells.

Free pyrimidine nucleotide pool of Ehrlich ascites-tumour cells. Characteristics related to quantitative studies of RNA metabolism

Ehrlich ascites-tumour cells incubated in a mineral medium supplemented with glucose and glutamine intensely incorporated labelled uridine into free nucleotides and RNA. Detailed kinetic studies of the labelling of total acid-soluble pools of uridine and cytidine nucleotides and of RNA under standard conditions and after chase with non-radioactive uridine were carried out. The relative distribution of the radioactivity between the individual uridine phosphates as well as their total cellular contents were also determined under standard and chase conditions. The labelling kinetics of the mononucleotides and RNA by radioactive uridine and orotate were compared, and some turnover parameters were estimated by mathematical analysis of a steady-state model. The following conclusions were drawn. (1) The observed chase effect of addition of non-radioactive uridine is due to a rapid expansion of the acid-soluble uridine nucleotide pool and consequently a severalfold lowering of its specific radioactivity. (2) The free uridine nucleotides are partly separated into a large and a small compartment, labelled preferentially by exogeneous orotate and uridine respectively. (3) The half-life of the mononucleotides, at least those located in the smaller compartment, is a few minutes only, owing to a rapid exchange with the uridylic acid residues of unstable RNA species. (4) This process of reutilization of RNA-degradation products accounts for 85% (a minimum estimate) of the overall turnover of the nucleotide pool in resting cells.

Evidence for compartmentation of uridine nucleotide pools in rat hepatoma cells

Interaction between the de novo and salvage pathways of pyrimidine metabolism was studied in a line of rat hepatoma cells by co-labelling with [14C]-uridine and [3H]orotate. A difference in the ratio of 14C/3H between CTP and UTP in acid-soluble nucleotide pool was reflected in the corresponding ratios in CMP and UMP in RNA, with uridine labelling cytidine nucleotides relatively more effectively than orotate. These results are not compatible with the concept of a single UTP pool, and a new model for pyrimidine anabolic pathways, based on compartmentation of de novo from salvage pathways, is proposed.

ADP-ribosylated histone H1 from HeLa cultures. Fundamental differences to (ADP-ribose)n-histone H1 conjugates formed into vitro

ADP-ribosylated histone H1 was isolated from intact HeLa cells grown for 24 h with[3H]-adenosine and compared with ADP-ribosylated histone H1 synthesized from [3H]NAD by isolated HeLa nuclei. Most (ADP-ribose)n-histone H1 conjugates formed in vivo carried single ADP-ribose units, less than one fourth of the total ADP-ribose residues being in the form of oligomeric or polymeric chains. (ADP-ribose)n linked to H1 in vivo was not released by neutral NH2OH to a significant extent. Alkali treatment (pH 10.5) liberated most but not all of the ADP-ribose residues which may indicate the existence of a new type of linkage so far found only in conjugates isolated from intact tissue. No ADP-ribosylated histone H1 complex of higher molecular weight ('H1 dimer') could be detected in intact cells. By contrast, isolated HeLa nuclei formed ADP-ribosylated histone H1 which contained predominantly polymeric ADP-ribose residues. The (ADP-ribose)n residues were linked by NH2OH-sensitive and by NH2OH-resistant, alkali (pH 10.5) labile bonds, the majority of the conjugates appearing in the form of the higher-molecular-weight complex. A comparison with the ADP-ribosylated non-histone proteins indicated that histone H1 formed in vivo carried less than 2.5% of the total protein-bound ADP-ribose residues and less than 1% of the protein-bound ADP-ribose synthesized in vitro.

Separate pyrimidine-nucleotide pools for messenger-RNA and ribosomal-RNA synthesis in HeLa S3 cells

Kinetic analyses of mRNA and 28-S RNA labeling [3H]uridine revealed distinctly different steady-state specific radioactivities finally reached for uridine in mRNA and 28-S RNA when exogenous [3H]uridine was kept constant for several cell doubling times. While the steady-state label of (total) UTP and of uridine in mRNA responded to the same extent to a suppression of pyrimidine synthesis de novo by high uridine concentrations in the culture medium, uridine in 28-S RNA was scarcely influenced. Similar findings were obtained with respect to labeling of cytidine in the various RNA species due to an equilibration of UTP with CTP [5-3H]Uridine is also incorporated into deoxycytidine of DNA, presumably via dCTP. The specific radioactivity of this nucleosidase attained the same steady-state value as UTP, uridine in mRNA and cytidine in mRNA. The data indicate the existence of two pyrimidine nucleotide pools. One is a large, general UTP pool comprising the bulk of the cellular UTP and serving nucleoplasmic nucleic acid formation (uridine and cytidine in mRNA, deoxycytidine in DNA). Its replenishment by de novo synthesis can be suppressed completely by exogenous uridine above 100 muM concentrations. A second, very small UTP (and CTP) pool with a high turnover provides most of the precursors for nucleolar RNA formation (rRNA). This pool is not subject to feedback inhibition by extracellular uridine to an appreciable extent. Determinations of (total) UTP turnover also show that the bulk of cellular RNA (rRNA) cannot be derived from the large UTP pool.

Cellular RNA and influenza-virion RNA are synthesized from different pyrimidine-nucleoside-triphosphate pools in chick-embryo cells

Chick embryo cells infected with an influenza A (fowl plague) virus have been labelled with (3H)-uridine for different lengths of time. Virion RNA and cellular RNA have been separated by specific hybridization with a surplus of unlabelled viral complementary RNA and RNase digestion. The ratio of the specific radioacticity in the UMP and CMP moieties of both types of RNA has been determined. Since the rate of approach to equilibrium of CMP to UMP labelling of both types of RNA is completely different it is concluded that cellular and virion RNA are synthesized using different pyrimidine nucleoside triphosphate pools.

Determination of mRNA half-life in HeLa cultures by a poly(A)-independent direct analysis of specific radioactivity of mRNA

mRNA specific radioactivity in HeLa cultures exposed to (3H)uridine (10 muM) was determined directly by a highly selective poly(A)-independent method which we have described previously. Neither uridine in mRNA nor UTP approached the specific radioactivity of the exogenous (3H)uridine, but attained steady-state specific radioactivities which remained a third below the value of the added precursor. Using the labeling data for the evaluation of mRNA turnover, previously described by Greenberg, mRNA half-life in exponentially growing HeLa cultures was found to be 0.87 times the cell doubling time. Decay curves of mRNA in prelabeled cultures were in accordance with these values (half-life equals 0.79 times the cell doubling time) when corrected for growth and also for "reutilization" which was accomplished by relating uridine labeling in mRNA to UTP specific radioactivity. The experiments showed that an exact evaluation of mRNA turnover is possible only when the following points are taken into account. a) A constant supply of exogenous labeled uridine must be provided to guarantee a constant specific radioactivity of UTP. b) Labeling of CTP and of cytidine in RNA are delayed when compared with UTP and uridine in RNA. Corrections for cytidine labeling in RNA are therefore required. c) As rRNA approached a definitely lower steady-state specific radioactivity than mRNA, mRNA specific radioactivities must be determined directly (i.e. by radioactivity and absorbance at 260 nm in isolated mRNA fractions) in order to evaluate true turnover of this RNA species.