MOLECULAR FACETS OF MITOTIC REGULATION, I. SYNTHESIS OF THYMIDINE KINASE

H-THYMIDINE DERIVATIVE POOLS IN RELATION TO MACRONUCLEAR DNA SYNTHESIS IN TETRAHYMENA PYRIFORMIS

The formation of a soluble H³-thymidine derivative pool has been examined in Tetrahymena pyriformis as a function of macronuclear DNA synthesis during the cell life cycle. An autoradiographic technique which allows the detection of water-soluble materials within a cell has shown that these cells do not take up and retain exogenous H³-thymidine during G1 or G2. Uptake of H³-thymidine is restricted to the S period of the cell cycle. Additional autoradiographic experiments show, however, that a soluble pool of H³-thymidine derivatives persists from the end of one DNA synthesis period to the beginning of the next synthesis period in the subsequent cell cycle. Since this persisting pool cannot be labeled with H³-thymidine, the pool does not turn over during non-S periods.

CELL DIVISION AND DNA SYNTHESIS IN TETRAHYMENA PYRIFORMIS DEPRIVED OF ESSENTIAL AMINO ACIDS

The question of amino acid requirements for DNA synthesis and cell division has been studied in Tetrahymena pyriformis by depriving cells of histidine and tryptophan at defined stages in the interdivision interval. Deprivation any time before DNA synthesis does not prevent the initiation of such synthesis but completely inhibits the following division and limits the increase in DNA, as measured microspectrophotometrically, to 20 per cent. H³-thymidine added to the medium is not incorporated during the 20 per cent increase. Deprivation after DNA synthesis is initiated does not prevent the continuation (to completion) of DNA synthesis, and cell division ensues. H³-thymidine added to the medium under these conditions is incorporated into macronuclear DNA. The data indicate that some amino acid-dependent event occurs, about the time of the beginning of the DNA synthesis period, which is not essential for initiation of DNA synthesis but which is essential for the maintenance of synthesis once it has begun. These results are further discussed in terms of enzymes required to convert thymidine (and possibly the other three deoxyribonucleosides) to the immediate precursor of DNA synthesis.

Immunological Characterization of a Tapetal Protein in Developing Anthers of Lilium longiflorum

Antiserum was raised in rabbits against a lily (Lilium longiflorum) anther-specific protein (LLA-15). Monospecific anti-LLA-15 antibodies were prepared to investigate the distribution of LLA-15 during anther development in a variety of flowering plants. Immunoblot analyses of total protein from floral and vegetative organs confirmed that LLA-15 or LLA-15-like proteins accumulated to detectable levels only in a discrete stage of anther development. In situ localization using anti-rabbit immunoglobulin G conjugated with gold particles confirmed that LLA-15 was specifically localized in the tapetal tissue of lily anthers. The maximal level of LLA-15 was strictly coincident with the peak of tapetal secretory functions. Immunoblots of two-dimensional polyacrylamide gels of lily anther proteins indicated that the seven LLA-15 isoforms ranged from isoelectric point 5.6 to 6.1. In vitro translation of lily anther mRNAs showed that four of these isoforms were primary products, the additional three being a result of posttranslational processing of the primary translation products.

Growth of anthers in Lilium longiflorum : A kinematic analysis

The post-initiation growth of 64 anthers (1.1-17.4 mm long) in Lilium longiflorum Thumb. was examined by time-lapse marking experiments in combination with serial sections and the scanning electron microscope. Each anther was characterized by spatial and temporal variation in growth rate. Larger anthers had two, and occasionally three, series of peaks and troughs in local growth rate. Regions of negative growth rate were frequently encountered. When observed over several days, the growth maxima and minima were found to move basipetally as a waveform down the length of the anther. The wavelength was longer in taller anthers; amplitude and frequency were variable, and anthers of the same size were not always synchronous. Distribution patterns of cell division (and elongation, once division has ceased) recapitulate the growth data. Anther growth is a non-steady system, therefore, with growth centers constantly shifting. Implications for future studies in organ growth patterns are discussed.

Nucleo-cytoplasmic interactions in the regulation of thymidine phosphorylation in Acetabularia

The activity of dT kinase (EC. 2. 7. 1. 75) was estimated during the development of nucleate and anucleate cells inAcetabularia. Enzyme activity increases as the cap attains its maximum diameter (Bannwarth & Schweiger 1975). Regulation of the dT kinase activity is observed even in the absence of the cell nucleus. Twelve to fifteen days after enucleation dT kinase activity increase sharply as the cap reaches its maximum diameter and decreases 5 days later. The increase is inhibited by puromycin, chloramphenicol, rifampicin and ethidium bromide but not by cycloheximide. These findings indicate that protein synthesis on 70S organelle ribosomes is involved in the regulation of the dT kinase. Since activities of mixtures from low activity cell extracts and from high activity cell extracts are additive, it is concluded that the regulation is not mediated through an activator protein but rather throughde novosynthesis of the enzyme protein.

Affinity chromatography of thymidine kinase from a rat colon adenocarcinoma

Thymidine kinase from a transplantable colon adenocarcinoma, induced by 1,2-dimethylhydrazine and maintained in CDF rats, was purified by affinity chromatography using thymidine-3'(4-aminophenylphosphate) coupled to carboxyhexyl-Sepharose. Most of the contaminating protein passed through the column; non-specifically adsorbed protein was washed from the column by 0.1 M KC1 in 0.01 M Tris-HC1, 7.5. Thymidine kinase was eluted with 0.1 mM thymidine, 0.1 M KC1 in 0.01 M Tris-HC1, pH 7.5. The purified enzyme accounted for about 26% of the applied activity; the specific activity of the purified material (peak fraction) was 3,500 moles TMP formed per mg protein per 10 min., a 1,800-fold purification of the applied extract. The preparation is free of nucleoside phosphotransferase, but contains other protein impurities. Purification was completed in less than 1 hour, making this a useful procedure for isolation of this unstable enzyme.

Programmed synthesis of flagellar tubulin during cell differentiation in Naegleria

Amebae of Naegleria gruberi differentiate into flagellates when transferred from growth medium to non-nutrient buffer. This differentiation, which requires 48 min at 28 degrees C, is dependent on transcription and translation. Tubulin of the flagellar outer doublets comprises about 0.15% of the protein of flagellate, and only about 1-2% of the total tubulin. An antiserum to flagellar (outer-doublet) tubulin contains antibodies that react selectively with flagellar tubulin. Measurements using this antiserum have shown that 97-98% of the flagellar tubulin antigen appears during differentiation. The appearance of tubulin antigen is sensitive to actinomycin D and cycloheximide. Isotope dilution experiments using [35S]methione demonstrated that at least 70% of the flagellar tubulin is synthesized from amino acids during differentiation. Experiments using both the specific antiserum and isotopes have shown that flagellar tubulin synthesis begins about one-third of the way through differentiation, before any morphological change has occurred. These experiments demonstrate that most, if not all, of the flagellar tubulin is synthesized de novo during differentiation, and that cells selectively use a specific subpopulation of tubulin in assembling the outer doub)lets. The results bring into focus major unsolved questions about the synthesis and assembly of flagellar tubulin.

Programmed synthesis of tubulin for the flagella that develop during cell differentiation in Naegleria gruberi

Amebae of Naegleria gruberi differentiate into flagellates when transferred from growth medium to nonnutrient buffer. Experiments were performed to determine whether the tubulin that forms the flagellar microtubules pre-exists in amebae or is synthesized during differentiation. Amebae prelabeled uniformly with [(35)S]methionine were allowed to differentiate in the presence and in the absence of exogenous unlabeled methionine. In the presence of unlabeled methionine the flagellar tubulin contained only 30% as much [(35)S]methionine as in its absence. Thus at least 70% of the tubulin was synthesized de novo. Isotope dilution and pulse experiments showed that flagellar tubulin synthesis began one-third of the way through differentiation, before any morphological change had occurred. Flagellar tubulin antigen, as measured using a specific antiserum, also began to increase one-third of the way through differentiation and increased 35- to 55-fold during the course of differentiation. These experiments demonstrate that most if not all of the flagellar tubulin is synthesized de novo during differentiation.

Temporal order in mammalian cells. I. The periodic synthesis of lactate dehydrogenase in the cell cycle

Chinese hamster cells were synchronized by the Colcemid-selection system. In cells with a division cycle time of 11.5-12 hr, the activity of the enzyme lactate dehydrogenase (LDH) underwent marked oscillations with a 3.5-hr period. Precipitation of labeled LDH enzyme with specific antibody indicated that the enzyme activity changes were the result of intermittent enzyme synthesis and relatively constant degradation. Inhibition of normal DNA replication with 4 mM of thymidine, while reducing the amount of new enzyme synthesized, did not prevent oscillations from occurring. Similarly, actinomycin D (AcD) added at the time of synchronization allowed some new enzyme synthesis to proceed in an oscillatory manner. LDH synthesis went on at nearly normal rates when AcD was added in the middle of S phase. However, addition of cycloheximide to cultures at any time in the cycle caused an immediate drop in levels of activity and in enzyme protein. The half-life of LDH, calculated either from loss of enzyme activity or precipitable radioactivity in cycloheximide-treated cultures, was between 2 and 2.5 hr.

Inhibition of thymidine kinase activity and deoxyribonucleic acid synthesis in L cells infected with the meningopneumonitis agent

The activities of enzymes related to deoxyribonucleic acid (DNA) synthesis were studied in uninfected L cells and in L cells infected with Chlamydia psittaci (strain meningopneumonitis). The meningopneumonitis agent multiplied normally but failed to induce the synthesis of thymidine kinase in LM (TK(-)) cells which contain no thymidine kinase in the uninfected state. It was concluded that this microorganism has no thymidine kinase of its own and that it does not depend on the functioning of the host enzyme for synthesizing its DNA. Exposure of clone 5b L cells to the meningopneumonitis agent was followed by a decline in their thymidine kinase activity to nearly zero levels, whereas the levels of uridine kinase and thymidylate synthetase remained unchanged. Inhibition of thymidine kinase activity in L cells occurred soon after infection and required new protein synthesis by the meningopneumonitis agent. This inhibition occurred before inhibition of host DNA synthesis, but it was not an essential prelude to the latter inhibition. On the basis of this and previous investigations and in light of present knowledge of the mammalian cell cycle, it was postulated that the meningopneumonitis agent inhibits macromolecular synthesis in L cells by preventing the initiation of a new cell cycle.

Nucleic acid and protein synthesis associated with the induction of nitrate reductase activity in radish cotyledons

1. RNA and protein synthesis was studied during the incubation of excised radish cotyledons in nitrate, conditions that induced nitrate reductase activity in the tissue. 2. Synthesis of total RNA and protein, as measured by the incorporation of radioactive precursor, was significantly stimulated in the presence of nitrate (compared with chloride control), but was decreased in the presence of ammonium nitrate, which induced higher enzyme activity. 3. Synthesis of RNA and protein was required for induction of enzyme activity, as determined by using the inhibitors actinomycin D, puromycin and cycloheximide. 4. On the basis of 5-fluorouracil inhibition, the synthesis of only DNA-like RNA was required for induction, but no differences, either quantitative or qualitative, were observed in DNA-like RNA synthesis in the presence or absence of induction. 5. A 100-fold purification of the nitrate reductase activity showed no increase in nitrate reductase protein, nor any increased incorporation of radioactive precursor into nitrate reductase protein in the induced versus the control system. Such results suggested that the protein synthesis required for induction may be for a protein other than nitrate reductase.

Inducibility of thymidine kinase by thymidine as a function of interphase stage

Thymidine may act as an inducer of thymidine kinase activity in cells of higher plants. The general response is demonstrable in a randomly developing cell population such as is found in germinating wheat embryos. If a synchronously developing cell population is studied, however, potentially inducible cells are found to be susceptible to the inductive effect of thymidine only during about 10 per cent of the G1 period, and close to the interval when thymidine kinase activity normally appears.