Vaccinia virus structural polypeptide derived from a high-molecular-weight precursor: formation and integration into virus particles

Polypeptide 4a, a major vaccinia structural polypeptide which was previously shown to form from a high-molecular-weight precursor is made after the period of viral deoxyribonucleic acid (DNA) synthesis. Pulse-chase experiments demonstrated that a period of 1 to 2 hr is required for a 50% conversion of precursor to product. The rates of incorporation of polypeptides into virus particles were examined. The kinetics of incorporation of labeled 4a and other major structural polypeptides into virus particles were similar, despite the additional time required for the formation of 4a from its precursor. Furthermore, 4a was present exclusively in a particulate form at all times examined. Both observations suggested that cleavage of the precursor occurs after, or immediately prior to, association with developing virus particles. Polypeptide P4a was previously identified as the probable precursor of 4a and is not ordinarily found in detectable amounts in virus particles. Under conditions in which breakdown of P4a was inhibited by adding rifampin or amino acid analogues after the period of viral DNA synthesis, isolated virus particles contained significant amounts of this polypeptide. Further analysis showed that P4a was localized within the virus core, which is also the site of 4a. Synchronization of virus assembly after the removal of rifampin was shown to be useful for studying the integration of polypeptides into a particulate fraction of the cytoplasm.

Interruption by Rifampin of an early stage in vaccinia virus morphogenesis: accumulation of membranes which are precursors of virus envelopes

Assembly of vaccinia virus envelopes and immature vaccinia particles was interrupted in HeLa cells treated with rifampin (rifampicin). The primary action of rifampin on vaccinia morphogenesis appeared to occur during the stage of envelope formation. When envelopes and immature particles were already present, maturation could continue, even in the presence of rifampin. It was demonstrated that the trilaminar membranes of irregular contour which accumulate in the presence of rifampin are precursors of virus envelopes. When rifampin was removed under controlled conditions, synchronous transitions were observed as the precursor membranes rapidly converted into uniformly curved envelope units with a 10- to 12-nm coat on the convex surface. These experiments provided an opportunity to examine the sequence of some early events in vaccinia morphogenesis. Initially, nascent envelopes remained in clusters around dense viroplasm. Large numbers of single immature particles appeared within 10 min. Nucleation of immature particles was the first evidence of core differentiation and began within 5 to 10 min. Development of lateral bodies and modeling of the biconcave cores was observed within 30 min, and structurally mature virions were present by 2 hr after the removal of rifampin. High resolution autoradiography showed that viral deoxyribonucleic acid, which labeled with (3)H-thymidine during rifampin treatment, was incorporated by the mature vaccinia which formed after rifampin was removed. Concentration of the viral deoxyribonucleic acid in core material evidently occurred after envelope assembly, probably coincident with nucleoid formation. Cytoplasmic crystalloid bodies accumulated during rifampin treatment; they appeared morphologically identical to vaccinia nucleoids and were heavily labeled by (3)H-thymidine.

Formation of a vaccinia virus structural polypeptide from a higher molecular weight precursor: inhibition by rifampicin

A vaccinia virus core polypeptide, with a molecular weight of 76,000 and a relative deficiency in tryptophan, was shown by pulse-chase experiments to form from a precursor. The latter may be a rapidly labeled, 125,000-molecular weight, tryptophan-deficient, virus-induced polypeptide, which diminished in quantity during the chase period and was barely detectable after two to three hours. Rifampicin completely prevented the formation of the core polypeptide without inhibiting the synthesis of the precursor. A rifampicin-resistant vaccinia mutant was used to demonstrate the specificity of this effect. The sequence of events after the removal of the drug suggested that cleavage of the precursor occurs during the formation of the virus core. Rifampicin appears to act by interrupting earlier maturational events which precede the formation of the core polypeptide.

Synthesis of vaccinia viral proteins in cytoplasmic extracts. II. Identification of early and late viral proteins

The synthesis of vaccinia viral proteins has been studied in a cell-free system prepared from vaccinia virus-infected HeLa cells. The radioactively labeled proteins were identified as viral proteins by immunodiffusion, disc gel electrophoresis, and disc gel-immunoelectrophoresis. The cytoplasmic extracts, obtained from infected cells at different times during viral replication, synthesized the corresponding "early" or "late" viral proteins.

Synthesis of vaccinia viral proteins in cytoplasmic extracts. I. Incorporation of radioactively labeled amino acids into polypeptides

Polypeptide synthesis has been studied in cell-free systems prepared from vaccinia virus-infected and uninfected HeLa cells. Cytoplasmic extracts containing endogenous messenger ribonucleic acid were used. Amino acid incorporation into hot trichloroacetic acid-precipitable material was linear for 15 to 20 min at 37 C. The initial rate of protein synthesis was approximately 15% of the rate in intact cells. Optimal conditions for polypeptide synthesis were similar in cell-free systems prepared from infected or uninfected cells. Requirements for an energy source and Mg(++) were demonstrated. The optimal Mg(++) concentration was 4 to 5 mm. Ribonuclease, puromycin, and cycloheximide were inhibitory. The molecular weights of the polypeptides labeled in the cell-free systems, as determined by gel filtration in 5 m guanidine hydrochloride, ranged from 16,000 to above 68,000. Polyacrylamide gel electrophoresis indicated that the polypeptides labeled in cell-free extracts of uninfected and infected cells were different. The latter closely corresponded in electrophoretic mobility with the viral polypeptides made in intact, infected cells.

ALGAE OF TWO SOMERSETSHIRE POOLS STANDING CROPS OF PHYTOPLANKTON AND EPIPELIC ALGAE AS MEASURED BY CELL NUMBERS AND CHLOROPHYLL A

Standing crops measured as cell numbers and as chlorophyll a content of phytoplankton and epipelic algal communities (those free-living on sediment surfaces) in 2 small water bodies of contrasted nutrient status were measured for 27 months. Mean yearly crops, on an areal basis, of phytoplankton were 4-13.4 times greater than those of epipelic algae in the nutrient-rich pool, but were only 0.65-1.8 times as great in the nutrient-poor pool, which, however, was shallower than the rich pool. The role of cells < 5 μ, in diameter in the composition of standing crops was minor. Effects of phytoplankton in limiting growth of epipelic algae by light attenuation are shown. Compilation of available data shows a direct relationship between epipelic algal crops in various water bodies and nutrient-status of the ambient water, similar to that already established for phytoplankton.

Sequential protein synthesis following vaccinia virus infection

Inhibition of HeLa cell protein synthesis and the sequential synthesis of viral proteins were followed by pulse-labeling infected cells with (14)C-phenylalanine. Proteins were resolved by polyacrylamide gel electrophoresis. The viral origin of native proteins was confirmed by immunodiffusion. The inhibition of host protein synthesis and the synthesis of early viral proteins occur 1 to 3 hr after infection. This early sequence of events also occurs in the presence of 5-fluorodeoxyuridine, an inhibitor of deoxyribonucleic acid synthesis. Other viral proteins are synthesized at a later time. Those proteins which are not made in the absence of viral deoxyribonucleic acid synthesis can be further subdivided into intermediate and late classes. The intermediate protein is synthesized before the late proteins but does not appear to be a precursor of them. Many more viral polypeptides were resolved by polyacrylamide gel electrophoresis after solubilization of the entire cytoplasmic fraction with sodium dodecyl sulfate. Virion and nonvirion proteins were identified. Kinetic experiments suggested that certain structural proteins as well as certain nonstructural proteins are made early, whereas others of both classes are made primarily at later times.

Inhibition of HeLa cell protein synthesis by the vaccinia virion

HeLa cell protein synthesis is rapidly suppressed after infection with purified vaccinia virus. This was measured in three ways. (i) In the presence of 5 mug of actinomycin D per ml, viral protein synthesis was prevented and the decline in host protein synthesis was measured directly. (ii) Virus particles irradiated with 800 ergs or more of ultraviolet (UV) light per mm(2) are defective in their ability to initiate viral protein synthesis, but they still inhibit host protein synthesis. After addition of UV-irradiated virus, the decline in host protein synthesis was measured. (iii) Polyacrylamide gel electrophoresis was used to distinguish between host- and virus-induced proteins. The following results were obtained. (i) The inhibition of HeLa cell protein synthesis begins within 20 min after infection with purified vaccinia particles. Greater than 95% inhibition occurs within 1 to 4 hr after infection, depending on the viral multiplicity used. (ii) The synthesis of viral ribonucleic acid or viral protein is not required for the inhibition of host protein synthesis. (iii) The ability of the virus particles to inhibit cell protein synthesis is lost after heat or detergent treatment. (iv) The ability of the virus particles to inhibit cell protein synthesis is retained after UV-irradiation. (v) Vaccinia viral protein synthesis in preinfected cells is resistant to the effects of superinfection with UV-irradiated vaccinia particles. (vi) Inhibition of cell protein synthesis is complete and does not involve the continued synthesis of small polypeptide fragments. (vii) A decrease in the size of host polyribosomes rapidly follows infection with vaccinia virus. The results are interpreted as a selective effect of some constituent of the vaccinia virus particle or virus-activated host enzyme on host protein synthesis at a level beyond that of transcription.

The repression and induction by thyroxin of hemoglobin synthesis during amphibian metamorphosis

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