The relationship between group C adenovirus tumor antigen and the adenovirus single-strand DNA-binding protein

The group C adenoviruses code for a single-strand specific DNA-binding protein of molecular weight 72,000 daltons which is synthesized at early times after productive viral infection. Experiments were designed to determine whether this single-strand specific DNA-binding protein was expressed in adenovirus tumors and transformed cells. Two independently derived preparations of anti-sera from hamsters bearing group C adenovirus tumors were tested for antibody against the single-stranded DNA-binding proteins. One antiserum contained antibodies that reacted with these DNA-binding proteins, while the second antiserum did not contain detectable levels of antibody. Five adenovirus type 2 transformed rat cell lines were tested for the presence of the single-stranded specific DNA-binding proteins. Two of the five transformed cells expressed detectable levels of this protein. These results indicate that the group C adenovirus single-strand specific DNA-binding proteins are expressed in some, but not all, adenovirus tumors and transformed cell lines. Those transformed cell lines (type 2) containing a portion of the adenovirus genome designated by the Eco R-I-B restriction enzyme fragment express the single-strand specific DNA-binding proteins. Those cell lines missing this Eco R-I-B fragment do not contain this viral protein. Other experiments have located the structural gene of the signle-strand specific DNA-binding protein in the Eco-R-I-B DNA fragment, indicating that when this gene is present in a transformed cell, it is expressed.

Cytoplasmic microtubules in tissue culture cells appear to grow from an organizing structure towards the plasma membrane

A structure which appears to organize cytoplasmic microtubules in interphase mouse 3T3 cells can be visualized by immunofluorescence microscopy. Purified monospecific antibody against homogeneous tubulin from brain visualizes, in addition to cytoplasmic microtubules, a cytoplasmic polar structure as the focal point from which the microtubules seem to radiate. The structure is preserved after treatments that depolymerize cytoplasmic microtubules, i.e., exposure of cells to mitotic drugs or to low temperature. When cells recover from these treatments one end of each microtubule organizing structure acts as a nucleating center from which cytoplasmic microtubules grow toward the plasma membrane. Thus cytoplasmic microtubules assemble in vivo in an ordered unidirectional manner, and therefore the cell must be able to avoid the assembly of unwanted, unoriented, and disconnected microtubules. These results suggest that the assembly of tubulin into microtubules is regulated in vivo.

Simian virus 40 gene A function and maintenance of transformation

Transformants have been isolated after infection of rat embryo cells at 33 C with either wild-type simian virus 40 or with the temperature-sensitive gene A mutants, tsA7 and tsA28. Examination of properties usually associated with transformation such as growth in 1% serum, growth rate, saturation density, and morphology show that these properties are temperature dependent in the tsA transformants characterized, but are not temperature dependent in the wild-type transformants that have been examined. In the most thoroughly characterized tsA transformants the expression of T antigen also appears to be temperature dependent. These data suggest that an active A function is required for the maintenance of transformation in these cells. In the lytic cycle, the A function is involved in the initiation of DNA synthesis. Thus transformation by simian virus 40 may be the direct consequence of the introduction of the simian virus 40 replicon and the presence of its DNA initiator function, which causes the cell to express a transformed phenotype.

Patterns of organization of actin and myosin in normal and transformed cultured cells

The patterns of distribution of intracellular actin and myosin were examined by specific immunofluorescence in a series of normal, simian-virus-40-transformed, and revertant cell lines of rat and mouse origin. A consistent correlation was found between sensitivity to anchorage-dependent growth control and the presence of large, thick sheaths of actin-containing material. The presence of these sheaths was temperature-dependent in a rat line transformed by a temperature-sensitive mutant in the complementation group A of the oncogenic virus simian virus 40.

SV40: T antigen, the A function and transformation

T antigen, a protein found in the nuclei of SV40-transformed and -infected cells, can be purified and characterized. Our data suggest that T antigen in transformed cells is found at least in part in a large 22S complex, suggesting that this 22S form may contain T antigen complexed with other viral or host proteins. We prefer the assumption that T antigen is correlated with the A function, although a definitive in vitro proof for this hypothesis is still missing. Mutants in the A function most likely interfere with the maintenance of transformation, a result inviting the interpretation that the A function may indeed govern DNA synthesis of the transformed cell or influence the cell's ability to pass from G1 (or G0) to S. Thus transformation by SV40 may be a direct consequence of the introduction of the SV40 replicon and the presence of its initiator function which is used by the transformed cell.

Nonsense suppression in a multiauxotrophic derivative of Escherichia coli 15T-: identification and consequences of an amber triplet in the deoxyribomutase gene

Previously, arginine revertants of Escherichia coli WWU, a derivative of E. coli 15T(-), have been subdivided by two independent methods: (i) the streak morphology on nutrient agar, and (ii) the pattern of phage growth using amber and ochre mutants of bacteriophage T4. In the first assay, revertants were subdivided into two classes according to the appearance of streaks after incubation on nutrient agar, a thick, even line of growth defining normal revertants and a thin, irregular line defining aberrant revertants. In the second assay, revertants were classified by the suppressors they contained. The present work demonstrates that revertants containing an amber suppressor show the aberrant morphology and are also able to catabolize thymidine for energy and carbon. This is in contrast to the parent WWU containing no suppressor, which shows a normal morphology and cannot utilize thymidine as an energy source. Revertants containing no suppressor, isolated specifically for their ability to catabolize thymidine, show an aberrant morphology. Together, these results indicate that the aberrant morphology results from suppression of an amber triplet in a gene of the thymidine catabolic pathway. Enzyme assays show the amber triplet to be in the gene specifying deoxyribomutase. It is suggested that the aberrant arginine revertants are analogous to high thymine-requiring mutants and that, in general, high and low thymine-requiring mutants differ from one another in their ability to catabolize deoxyribose-1-phosphate.