Bacterial inactivation of transforming deoxyribonucleate

Transformation in Agmenellum quadruplicatum

A DNA-mediated transformation system for the blue-green alga Agmenellum quadruplicatum, strain PR-6, is described and characterized for DNA concentration dependence, dependence on time of exposure to DNA, phenotypic expression, sensitivity to various enzymes, and competence. The stability of the transformants has been investigated, and genetic backcross and selfing experiments have been performed. This system fulfills all of the criteria established for the well-characterized transformation systems in heterotrophic bacteria and demonstrates significant similarities to at least one of these systems for all characteristics examined. The efficiency of transformation is high. This system fills a need for a well-characterized genetic system in an oxygen-evolving photoautotroph. We have used it to transform a strain with a mutational lesion in assimilatory nitrogen metabolism to a wild-type genotype.

Transformation in cyanobacteria

The lack of any known transduction or indigenous conjugation systems has left transformation as the major means for genetic manipulations in cyanobacteria. Studies of transformation in cyanobacteria generally have dealt with one of two distinct areas. The first area is genomic transformation where internalized donor DNA recombines with chromosomally located genes. Chromosomal transformation can be a powerful tool for genetic mapping and mutagenesis. The second area is plasmid transformation where internalized plasmid donor DNA becomes established as an independent replicon in the recipient cyanobacterium. This second area has received a great deal of attention because it allows the generation of merodiploids for studies of genetic regulation and control and because it potentially allows the expression of foreign genes in an oxygenic photoautotroph. This article will attempt to describe the development of our current understanding of these two types of genetic transformation in cyanobacteria.

Competence related proteins in the supernatant of competent cells of Bacillus subtilis

We report that centrifugation at relatively high g-forces reduces the ability of competent cells of Bacillus subtilis to bind and take up DNA, and to be transformed. The centrifugation supernatant from competent cells restores this reduction of competence; the supernatant from non-competent cells is inactive. Phosphocellulose chromatography of centrifugation supernatants from radioactive competent cultures gave rise to six sharp peaks, together, these were shown by subsequent SDS polyacrylamide gel electrophoresis to contain over 60 different polypeptide bands. Peak II, which showed competence restoring activity, produced three polypeptides. When these bands were further examined, one of these exhibited DNA binding activity and the other two each contained a different endonuclease. Competence restoring activity was not recovered from the SDS polyacrylamide gel of peak II. The three peaks from non-competent cultures produced altogether five faint bands in gel electrophoresis. None of these bands were similar to those found in peak II.

Interactions of competent Streptococcus sanguis (Wicky) cells with native or denatured, homologous or heterologous deoxyribonucleic acids

Competent cell-deoxyribonucleic acid (DNA) interactions were examined using tritium-labeled homologous or heterologous native or denatured DNAs and competent Streptococcus sanguis Wicky cells (strain WE4). The DNAs used were extracted from WE4 cells, Escherichia coli B cells, and E. coli bacteriophages T2, T4, T6, and T7. The reactions examined were: (i) total DNA binding, (ii) deoxyribonuclease-resistant DNA binding, and (iii) the production of acid-soluble products from the DNA. Optimal temperatures for the reactions were as follows: reaction (i), between 30 and 40 degrees C; reaction (ii), 30 degrees C; and reaction (iii), greater than 40 degrees C. The rates for the reactions (expressed as molecules of DNA that reacted per minute per colony-forming unit) did not vary greatly from one DNA source to another. With a constant competent cell concentration and differing DNA concentrations below a saturation level (from a given source), a different but constant fraction of the added DNA was cell bound, deoxyribonuclease resistant, and degraded to acid-soluble products. In experiments where the number of competent cells was varied and the DNA concentration was held constant, again essentially the same result was obtained. The extent of reactions (i), (ii), and (iii) depended upon the numbers as well as the source of DNA molecules applied to competent cells. Calcium ion essential for native DNA-cell reactions was also found essential for denatured DNA-cell reactions. Data obtained from competition experiments lead to the conclusion that competent WE4 cells contain specific sites for native as well as denatured DNAs.

Enchancement of streptococcal transformation yield by proteolytic enzymes

Trypsin and other proteolytic enzymes, added together with transforming DNA or during cell-DNA contact to competent cultures of several streptococcal strains, enchanced (10 to 600%) the yield of genetic transformation (stimulation). With few exceptions, the level of stimulation was high (over 100%) when competence was low (below 2%). Stimulation was caused by the action of an enzyme on competent cells and not on any other component of transformation mixture. The phenomenon occurred when the enzyme was added to the culture not earlier than 7 min before and not later than 5 min after the period of cell-DNA contact. The presence of trypsin during cell-DNA contact caused: (i) the alterations at cell surface, demonstrated by electron microscopy, increased release of 3H-amino acid-labeled material, and higher cell susceptibility to autolysis; (ii) the increase of both total and irreversible binding of DNA by the cells; and (iii) the decrease of early nucleolytic degradation of DNA by cells. These and other data point to the importance of a delicate balance of recipient cell's surface nuclease activities in the effectiveness of transformation process. It is also possible that trypsin eliminates an unknown cellular factor which obstructs DNA-cell receptors interaction.

Purification and properties of a manganese-stimulated endonuclease from Bacillus subtilis

An endonuclease stimulated by manganese or calcium ions was isolated from Bacillus subtilis. This enzyme attacked double- or single-stranded deoxyribonucleic acid from a variety of sources, including B. subtilis, and was purified from the material released into the medium during protoplast formation. The enzyme appeared as a single peak after glycerol gradient centrifugation and comprised approximately 30 to 35% of the protein in the most purified preparations, as estimated by gel electrophoresis. It had a molecular weight of about 46,000. The mode of action of the enzyme was endonucleolytic, and circular deoxyribonucleic acid was readily cleaved. The enzyme introduced a limited number of both double- and single-strand breaks into native deoxyribonucleic acid, generally yielding products of 1 X 10(6) daltons or more in size. The reasons for this limitation of cleavage were not clear. The activity of the enzyme was inhibited by low levels of Cu2+, Co2+, Hg2+, and Zn2+. It was also inhibited by high concentrations of NaCl. A role for this enzyme in bacterial transormation is suggested.

Cell surface-located deoxyribonucleic acid receptors in transformable pneumococci

We studied deoxyribonucleic acid (DNA) binding in transformable pneumococci. The relevant findings are as follows. (i) At least half of the DNA Molecules adsorbed to competent cells in the growth medium are attached to sites on the protoplast membrane. (ii) Most of the DNA bound to live competent cells in the presence of glucose is not released by moderate shear or by autolysin treatment. In contrast, most of the DNA adsorbed to competent cells in the absence of glucose is shear and autolysin sensitive. (iii) The presence of binding sites resembling in properties the sites in live competent cells can be demonstrated in wall-membrane complexes. Most of these sites are lost during preparation of cell walls and protoplasts. It is suggested that the DNA-binding site is a membrane component (protein?) Stabilized by polysaccharide (cell Wall) material. (IV) Mechanical or enzymatic damage to the cell wall or change in the ionic conditions can induce DNA binding (and surface-nuclease activity) in the incompetent pneumococci. However, such cells still show neither genetic transformation nor extensive nuclease-resistant binding of DNA. It is suggested that both competent and incompetent cells contain a large number of sequestered DNA-binding sites that can be unmasked by several experimental conditions. Induction of the competent state by the competence activator protein may involve an endogenous unmasking process.

Nucleolytic degradation of homologous and heterologous deoxyribonucleic acid molecules at the surface of competent pneumococci

Competent pneumococci can catalyze the rapid and quantitative degradation of extracellular deocyribonucleic acid (DNA) molecules through the activity of surface-located nucleases (endo- and, possibly, exonucleases as well). Both homologous and heterologous DNAs are degraded by a mechanism that seems to involve a cyclic process: (i) attachment of DNA to the cell surface followed by (ii) nucleolytic attack, and (iii) release to the medium. Processes (ii) and (iii) are both inhibited by ethylenediaminetetraacetate. Whereas surface nuclease activity is specific for competent cells, the bulk of this activity is not coupled to irreversible DNA uptake (deoxyribonuclease-resistant binding). Pneumococcal DNA treated with ultraviolet irradiation or nitrous acid (cross-linking?) is selectively impaired in the ability to irreversibly bind to competent cells, whereas reversible binding is normal.

Different nuclease activities in competent and noncompetent Bacillus subtilis

Competent and noncompetent cells of Bacillus subtilis were separated on the basis of their different buoyant densities. The two types of cells were compared with respect to their interactions with exogenous deoxyribonucleic acid(DNA). After exposure of DNA to the cells, the unadsorbed fraction of DNA molecules was examined. Both types of cells decreased the biological activity of this DNA, the inactiviation exerted by noncompetent cells being more severe than that exerted by competent cells. Sedimentation analysis of the inactivated DNA revealed that fragments of DNA are produced, owing mainly to the introduction of double-strand scissions. In addition to this fragmentation, the competent bacteria extensively digested the DNA exonucleolytically. This type of breakdown was specifically related to the competent state rather than to the state of low density. The exonucleolytic activity is, in all probability, associated with the cell envelope, because most of the activity is released into the medium when the cells are converted to protoplasts. At 37 C the competence-specific exonucleolytic breakdown started 2 to 3 min after the binding of DNA to the cells. In unfractionated cultures, breakdown may proceed until 70% of the total amount of DNA added has been made acid soluble. Nontransforming Escherichia coli DNA was also subject to exonucleolytic degradation; it seems unlikely,therefore, that this type of breakdown occurs as a consequence of recombination. Since ethylenediaminetetraacetate blocked both transformation by native DNA and the exonucleolytic breakdown of bound DNA, we suggest that the breakdown of DNA by competent cells fulfills an essential function in genetic transformation of B. subtilis.

Marker discrimination in deoxyribonucleic acid-mediated transformation of various Pneumococcus strains

The function responsible for discrimination among markers (point mutations) in Pneumococcus (hex) was traced back to the early strains used to demonstrate the chemical basis of transformation in the early 1940s. Those currently used laboratory strains failing to manifest this function arose from a single subline of the original strain. The function was also evident in other independently isolated strains including a number of different serological types. The hex function was not evident in transformation between heterologous pneumococcal strains probably as a result of the sensitivity of the function to saturation in the presence of deoxyribonucleic acid from closely related but nonisogenic strains.

Structure of deoxyribonucleic acid on the cell surface during uptake by pneumococcus

We exposed competent cells of Diplococcus pneumoniae to high-molecular-weight donor deoxyribonucleate (DNA) and examined the state of the DNA bound to them in forms sensitive to deoxyribonuclease I. The portion elutable with 5 M guanidine hydrochloride was shown to be native, of much lower molecular weight (4 x 10(6) to 5 x 10(6)) than the donor, and as active in further transformation as sheared DNA of the same size. The portion resistant to release by guanidine hydrochloride was also shown to be native and active in transformation. These results, along with previous ones, imply that the breaks produced outside the cell are not at genetically specific sites. Furthermore, it was found that entry past the cell barrier to deoxyribonuclease could occur at 0 C by a process sensitive to ethylenediaminetetraacetate.

Number of deoxyribonucleic acid uptake sites in competent cells of Bacillus subtilis

Two direct methods are presented for estimating the average number of deoxyribonucleic acid (DNA) uptake sites in competent cells of Bacillus subtilis from measurement of (14)C- or (3)H-thymine-labeled DNA uptake by competent culture. Advantage is taken of two facts: (i) effective contact between competent cells and transforming DNA molecules is established within a short time after mixing them together, and (ii) DNA molecules enter the competent B. subtilis cells in a linear fashion at a finite speed. From the number of DNA molecules initially attached to competent cells by brief exposure to transforming DNA in the first method or from the rate of DNA uptake by competent culture in the second method, the average number of DNA uptake sites is calculated to be 20 to 53 per competent cell.