Physical and biological features of polyoma virus mutants able to infect embryonal carcinoma cell lines

Three new polyoma mutants were selected for their ability to grow on the embryonal carcinoma cell line F9. These mutants share in common an insertion of two nucleotides, a thymine and an adenine, in the noncoding region located on the late side of the origin of replication. We have found that these insertions exist in all of the other polyoma virus mutants able to grow on F9 cells (Fujimura et al., Cell 23:809-814, 1981; Katinka et al., Nature (London) 290:720-722, 1981; K. Sekikawa and A. J. Levine, Proc. Natl. Acad. Sci. U.S.A. 78:1100-1104, 1981). The region containing these insertions could be folded into a stable secondary structure which included a guanine plus cytosine (G + C)-rich stem. The adenine and thymine were inserted in such a way that they maintained the palindrome in the G + C-rich stem and were complementary in the putative secondary structure that we present here. Another class of polyoma virus mutants selected on a multipotential carcinoma cell line (PCC4-Aza) were characterized by a more complex rearrangement (deletion and duplication) which occurred in the same region. This arrangement preserved the G + C-rich palindrome and also yielded a sequence which still allowed the folding of another type of stable secondary structure. The significance of these findings is discussed.     

Photosensitized splitting of thymine dimers in DNA by gene 32 protein from phage T 4

The binding of denatured DNA to the protein coded by gene 32 of phage T 4 is accompanied by a quenching of the fluorescence of the protein tryptophyl residues. Gene 32 protein also binds to UV-irradiated DNA and photosensitizes the splitting of thymine dimers. Thymine bases are regenerated by this photosensitized reaction both in double stranded and in heat denatured DNA. No photosensitized splitting of thymine dimers is observed when the complex formed by gene 32 protein with UV-irradiated DNA is dissociated at high ionic strength. These results are discussed with respect to the possible stacking interaction of tryptophyl residues of gene 32 protein with bases in single stranded DNA.     

INTERCELLULAR COMMUNICATION AND TISSUE GROWTH : III. Thyroid Cancer

Intercellular communication was examined in normal and cancerous isolated thyroids with an intracellular electrical technique. The cells of normal thyroid (rat, mouse, hamster, man) communicate, within any given follicle, through permeable junctions. The cells of a wide variety of thyroid cancers (rat, hamster) do not communicate to any detectable degree and have resting membrane potentials lower than those of normal cells.     

Intercellular communication and tissue growth

Summary Three cancer cell strains that fail to make permeable membrane junctions were tested for ability to transfer an endogenous hypoxanthine derivative from cell to cell. The cells of these strains, loaded with³H-hypoxanthine, were grown in contact with cells from a mutant line incapable of incorporating exogenous hypoxanthine. The transfer of the³H-hypoxanthine derivative to the mutant cells was determined by radio-autography and, in the same preparations, the presence of permeable membrane junctions was determined by intercellular fluorescein tracer diffusion and electrical measurement. The cells of the three strains showed no transfer of hypoxanthine derivative to contiguous mutant cells; the cells that make permeable junctions did show such transfer, under the same conditions.In contrast to this contact-requiring mode of transfer, a contact-independent transfer phenomenon was observed with these three cancer cell strains.