Influence of genetic markers and of the fusing agent polyethylene glycol on chromosomal gene transfer inSpiroplasma citri

Spiroplasma citri, a plant pathogenic molligute: relationships with its two hosts, the plant and the leafhopper vector

Spiroplasma citri, the type species of the genus Spiroplasma (Spiroplasmataceae, Mollicutes), is restricted to the phloem sieve tubes and transmitted by phloem sap-feeding insects, as is characteristic of the phytopathogenic mollicutes. The spiroplasmas are the only mollicutes showing motility and helical morphology, apparently mediated by a contractile fibrillar cytoskeleton bound to the inner surface of the spiroplasmal membrane. MreB genes, which are involved in cell-shape determination, have been identified in S. citri. Identified genes of other functional groups are those involved in the transmission of S. citri by the leafhoppers and genes coding for lipoproteins, including spiralin, bound to the outer surface of the spiroplasma membrane. S. citri mutants that are unable to use fructose induce only mild and delayed symptoms. Fructose utilization by the sieve tube-restricted wild-type spiroplasmas is postulated to deprive the companion cells of fructose, thereby impairing sucrose loading into the sieve tubes.

Gene transfer in Mycoplasma pulmonis

Experiments were undertaken to examine gene transfer in Mycoplasma pulmonis. Parent strains containing transposon-based tetracycline and chloramphenicol resistance markers were combined to allow transfer of markers. Two mating protocols were developed. The first consisted of coincubating the strains in broth culture for extended periods of time. The second protocol consisted of a brief incubation of the combined strains in a 50% solution of polyethylene glycol. Using either protocol, progeny that had acquired antibiotic resistance markers from both parents were obtained. Analysis of the progeny indicated that only the transposon and not flanking genomic DNA was transferred to the recipient cell. Gene transfer was DNase resistant and probably the result of conjugation or cell fusion.

Molecular biology of mycoplasmas

Although mycoplasmas lack cell walls, they are in many respects similar to the gram-positive bacteria with which they share a common ancestor. The molecular biology of mycoplasmas is intriguing because the chromosome is uniquely small (< 600 kb in some species) and extremely A-T rich (as high as 75 mol% in some species). Perhaps to accommodate DNA with a lower G + C content, most mycoplasmas do not have the "universal" genetic code. In these species, TGA is not a stop codon; instead it encodes tryptophan at a frequency 10 times greater than TGG, the usual codon for this amino acid. Because of the presence of TGA codons, the translation of mycoplasmal proteins terminates prematurely when cloned genes are expressed in other eubacteria, such as Escherichia coli. Many mycoplasmas possess strikingly dynamic chromosomes in which high-frequency changes result from errors in DNA repair or replication and from highly active recombination systems. Often, high-frequency changes in the mycoplasmal chromosome are associated with antigenic and phase variation, which regulate the production of factors critical to disease pathogenesis.

Fusion-mediated transfer of plasmids into Spiroplasma floricola cells

We have developed and characterized a system for the transfer of plasmids encapsulated in large unilamellar vesicles (LUV) into Spiroplasma floricola BNR1 cells. The approach is based on the ability of S. floricola-derived LUV to fuse with S. floricola cells. The fusion was continuously monitored by an assay for lipid mixing based on the dequenching of the fluorescent probe octadecylrhodamine B (R18) that was incorporated into LUV at self-quenching concentrations. The fusion was also evaluated by fluorescence-activated cell sorter measurements and by sucrose density gradient analysis. LUV-cell fusion occurred only in the presence of low concentrations (5%) of polyethylene glycol (polyethylene glycol 8000) and depended on temperature, the LUV/cell ratio, and divalent cations in the incubation medium. Throughout the fusion process, spiroplasma cells remained intact and viable. Under optimal fusion conditions, the plasmid pACYC, encapsulated in LUV by reversed-phase evaporation, was transferred into live S. floricola cells and expressed chloramphenicol acetyltransferase activity. The expression was transient with maximal chloramphenicol acetyltransferase activity observed after 6 h of incubation of the transfected cells.

Small unilamellar vesicles are able to fuse with Mycoplasma capricolum cells

We have investigated the fusion characteristics of intact Mycoplasma capricolum cells and small unilamellar vesicles (SUV). The rate and extent of fusion was monitored continuously by octadecylrhodamine B (R18) fluorescence dequenching assay, as well as by intracellular contents mixing, and by sucrose density gradient analysis. The fusion of SUV with M. capricolum cells was found to be dependent on poly(ethylene glycol) (PEG 8000), divalent cations in the medium, and on the cholesterol content of the lipid vesicles. Maximal levels of fusion were obtained with SUV containing 40 mol% cholesterol in the presence of 5% PEG. The rate and extent of fusion were affected by temperature, pH, osmotic pressure, and SUV/mycoplasma ratio. Under optimal fusion conditions, PEG did not increase the rate of exchange of either cholesterol or phospholipids between M. capricolum cells and SUV. Throughout the fusion process, M. capricolum cells remained intact as measured by the retention of [3H]thymidine-labeled components, and viable. M. capricolum cells were rendered nonfusogenic by treatment with glutaraldehyde (greater than 0.01%) or chlorpromazine (greater than 10 microM). Fusion was partially inhibited by treating the cells with the uncoupler CCCP (5 microM) or proteolytic enzymes, suggesting that a proton gradient across the cell membrane is required for the fusion, and that the cells possess proteinase-sensitive receptors that are responsible for a tighter contact with the lipid vesicles.