DNA isolated from Mycoplasmatales viruses MVL51 and MVGs51 was infectious when mixed with Acholeplasma laidlawii BNL-NaLR cells. Infectivity was destroyed by deoxyribonuclease but not by ribonuclease, Pronase, or specific antiserum to the virus. Host mycoplasma cells were only competent for transfection during late-log growth phase. The rates of the establishment of DNase insensitivity of viral DNA transfectants were similar to those of bacteriophage systems. The doseresponse curve for transfection suggested that an average of six molecules of DNA must interact with a cell in order to produce one infectious center. Mycoplasmatales virus DNA exhibited a low efficiency of infection; one infectious center required 4 X 106 virus equivalents of DNA.Mycoplasmas are a group of small prokaryotic cells (Order, Mycoplasmatales) bounded only by a single lipoprotein membrane. They are the smallest free-living cells: different mycoplasma species carry enough genetic information to only code for 600-1000 cistrons per cell (1). Previous attempts to study mycoplasma genomes have suffered from the lack of a system for performance of conjugation, transformation, or tranduction in these cells (2). Folsome (3) has reported that one mycoplasma species, Acholeplasma laidlawii A, was capable of binding in DNase-resistant form double-or single-stranded DNA of high molecular weight; however, the existence of transformation could not be demonstrated.Transfection, the infection of cells with isolated viral DNA, has been reported for several bacterial species: Escherichia coli (4, 5), Streptomyces kanamyceticus (6), Proteus (7), Bacillus species (8, 9), and Staphylococcus aureus (10). Infectious DNA has also been reported for many animal virus systems: polyoma (11), simian virus40 (12, 13), and Shope rabbit papilloma virus (14). This paper reports a transfection system in A. laidlawii by DNA from several Mycoplasmatales viruses. The lack of cell walls of the mycoplasmas means that this system may be similar to those of animal cells or bacterial protoplasts.
MATERIALS AND METHODSCells and Media. The mycoplasma used for virus propagation and as an indicator strain was A. laidlawii BN1-Na1R (NaiR). It is a nalidixic acid-resistant mutant of a strain, A. laidlawii BN1 (15). The isolation of Na1R and the tryptose broth and agar media used in these studies have been described (16). Cells were assayed as colony-forming units (CFU/ml) (17).Virus Strains. The two Mycoplasmatales viruses used were MVL51, isolated from a spontaneous plaque on a NaiR lawn, and MVGs51, isolated from a lawn of Mycoplasma gallisepticum A5969. Both viruses plaque on NaiR, but are distinguishable by their different growth rates on NaiR and their different inactivation rates by antiserum against MVL51. The viruses were assayed as plaque-forming units (PFU) on NaiR lawns.Preparation of Viruses. Viruses were propagated as described (16). Lawns were started by plating an 18-hr culture, which had been diluted 1:1 with fresh medium. 6-hr Lawns of NaiR on tryptose-agar pl...