Recombination Events in the Bacterial Genus Nocardia

Characterization of the mobilization determinants of pAN12, a small replicon from Rhodococcus erythropolis AN12

Bacteria belonging to the Gram-positive actinomycete species, Rhodococcus erythropolis, are diverse not only in terms of metabolic potentials but the plasmids they encode. It was shown previously that the R. erythropolis AN12 genome harbors a 6.3kb cryptic plasmid called pAN12, which is a member of the pIJ101 family of plasmids. Here we show that pAN12 is conjugatively mobilizable into other rhodococcal strains. A series of plasmid deletion constructs were tested for loss of mobility to identify the pAN12 cis-acting conjugation requirement. In this way, an approximately 700bp region was found to be required for plasmid transmission. A small 61bp element within this region confers mobility to an otherwise non-mobilizable plasmid. Unlike pIJ101, which encodes all necessary factors for transfer, pAN12 mobility is dependent on the presence of an AN12 megaplasmid, pREA400.

Rifamycins: strain improvement program

Rifamycins are primarily produced by Gram-positive bacterium Amycolatopsis mediterranei, which belongs to the order Actinomycetales. These antibiotics, apart from their application against pathogens of tuberculosis and leprosy, have also been found to be effective against several other pathogens including Mycobacterium avium and Pneumococcus. Because of the importance of rifamycin, the producer strain A. mediterranei has been genetically manipulated since 1957 in order to develop a strain that can either produce larger amounts of rifamycin or derivatives of rifamycin. In this article, the importance of the producer strain, traditional methods (mutations and recombination) of strain improvement, their limitations, and the development of a cloning vector and transformation methods that have made recombinant DNA techniques accessible for genetic manipulations of A mediterranei are discussed.

Identification of DNA involved in Rhodococcus chromosomal conjugation and self-incompatibility

Genes essential for Rhodococcus chromosomal conjugation were found to be located on an unstable genetic element in one of the strains investigated. From this element two segments of DNA could be identified which were involved in conjugation of Rhodococcus strains. One region, spread over 8 kb, was involved in the property of self-incompatibility. A second region, of about 3 kb, was essential for conjugation between ATCC 12674 and related nocardioforms.

The genetics of mycobacteria and mycobacteriophages - a review

The genus Mycobacterium, despite its medical importance, has so far received relatively little attention from bacterial geneticists. Nevertheless examples of the transfer of genes from one strain to another by means of transformation, phage-mediated transduction and direct cellular contact have been reported. The modification of strains by experimental or naturally occurring lysogeny has been studied in some detail and there is evidence that phage may contribute significantly to variation within species. Mycobacteriophages have been the subject of genetic analyses and have proved of value in the study of certain aspects of host-induced phage modification. In addition phages are being used to develop typing systems for use in epidemiological studies. It is evident that much of interest and value awaits discovery by means of genetic analysis of the mycobacteria and their phages and hopefully the next decade or so will bring great advances in this subject.

Possibilities of the conjugation process in mycobacteria

Results obtained when studying conjugation in mycobacteria by means of different methods are summarized. The method of conjugation on surface of a solid complete medium was tested with different auxotrophic mutants of different strains of Mycobacterium smegmatis. It was not possible to obtain positive results even by means of the above method. This was probably due to unsuitability of the chosen strains of Mycobacterium smegmatis. Preparation of the donor strain by transfer of the F factor from Escherichia coli F'ORF 1 ade+ lac+ pro+ to Mycobacterium phlei PA ade Stmr by means of sexduction is described. Frequency of the phenotype PA ade+ Stmr increased in the average by two and a half orders of magnitude with respect to the control, however, a further transfer from cultures of the cells ade+ Stmr to cells ade could not be demonstrated. Experiments aimed at transferring the R factor from strains Escherichia coli K-12 to Mycobacterium phlei were unsuccessful.

Genetic recombination in Nocardia mediterranei

A system of genetic recombination in Nocardia mediterranei ATCC 13685 is described. This strain produces a mixture of several rifamycin antibiotics. Using haploid recombinant selection and analysis procedures similar to those applied to Streptomyces coelicolor A3(2), 14 auxotrophic markers and 1 streptomycin resistance marker were located on a circular linkage map. The linkage map of N. mediterranei seems to be similar to that of S. coelicolor A3(2).

Limiting nutrients during mating as a means of increasing recombinant recovery in crosses of Nocardia erythropolis

Minimal medium supplemented with sub-optimal levels of parental auxotrophic requirements increases recombinant recovery from matings of Nocardia erythropolis more than 500-fold when compared with nutrient agar matings.

Genetic recombination in mycobacteria

Evidence for genetic recombination between Mycobacterium smegmatis strain Rabinowitchi (Rab) and strain Jucho or PM5 is presented. Backcrosses of recombinants by either parental strain indicated four different types of mating behavior, suggesting that the mycobacterial compatibilities are controlled by at least two different factors. No sex factor that transfers at a high frequency or that is sensitive to acridine dyes was detected. Analysis of segregation of unselected markers revealed that strain Jucho, or PM5, contributes the majority of alleles in almost all recombinants obtained from different selective media. Efforts to construct linkage maps for the markers employed failed because of ordering ambiguities. Mating medium containing streptomycin prevented genetic recombination when strain Rab was resistant to the antibiotic and Jucho, or PM5, was sensitive, but it did not prevent recombination when Rab was sensitive to streptomycin and Jucho, or PM5, was resistant. Very low frequency of recombinant formation was observed when Jucho, or PM5, had been treated with streptomycin, whereas recombinants were formed at fairly high frequencies when Rab had been treated with the antibiotic, suggesting that the roles of parental strains in zygote formation were not identical. The results suggest a polar transfer of genetic material from Rab to Jucho, or PM5, although an alternative possibility of cell fusion followed by exclusion could not be excluded.

Genophore homologies among compatible nocardiae

Deoxyribonucleic acid (DNA) reassociation analyses were employed to determine the molecular relationships between recombinable nocardiae. Analysis of the compatibility system of Nocardia erythropolis Mat-Ce and Mat-cE mating strains demonstrated the existence of extensive homology under both exacting and nonexacting conditions. Labeled N. erythropolis Mat-cE DNA reassociated equally as well with the Mat-Ce test DNA as with its own filter-bound DNA. However, the Mat-cE DNA bound only ca. 60% of the Mat-Ce DNA, when the latter was the reference. The existence of unique nucleotide sequences is postulated on the basis of these results as well as of aberrant segregation patterns which have been observed in certain class types of recombinants. Reassociation data reveal that recombinants representing the inheritance of different portions of each of the parental genomes have inherited the unique portion from the Mat-Ce parent. N. restrictus AY-B-226 exhibited little relatedness (11 to 32%), and N. globerula ATCC 9356 only slightly more (21 to 42%), to either of these mating strains at either exacting or nonexacting temperatures of incubation.

Genetic recombination in Micromonospora

Biochemical mutants were obtained from Micromonospora chalcea, M. purpurea, and M. echinospora by using ultraviolet radiation or nitrosoguanidine. Crosses carried out between complementary nutritional mutants of the same species showed positive genetic interaction. Data are reported which indicate that the interaction between the crossed strains is due to genetic recombination. No evidence for interspecific genetic recombination was found.

Inheritance of mating factors in nocardial recombinants

The segregation of mating loci with other unselected genes was analyzed in recombinants obtained from matings of Nocardia canicruria and N. erythropolis. The loci C/c and E/e control nocardial compatibility. Four mating genotype combinations were observed: cE, Ce, CE, and ce. Strains of N. erythropolis bear the genotype cE and strains of N. canicruria bear the Ce alleles. The CE recombinant mating type is capable of mating with both organisms, whereas the ce-containing recombinant is nonfertile. The E locus was found to segregate with StrA1 (streptomycin-resistance gene) on the N. erythropolis linkage group. The C locus appeared linked to PurB2 (purine-requiring gene) on the N. canicruria linkage group. A few observed recombinants were capable of further segregation of unselected characters after colonial purification, suggesting a possible heterogenomic condition or multiple rounds of mating. Prior treatment of recombinants with acriflavine failed to alter their compatibility or the frequency at which recombinants were recovered. The segregation pattern of the mating loci allowed for specific recombinant class types to be compatible.

Partial exclusion of the Nocardia erythropolis chromosome in nocardial recombinants

Segregation of genes specifying nutritional requirements and inhibitor resistance was analyzed in recombinants from crosses of Nocardia erythropolis with N. canicruria. With all employed characters as both selecting and nonselecting markers, a single linear linkage group was depicted with genes in the following order; leu-6 gly-4 purB2 tetA9 his-1 eryA7 strB2 ser-1 arg-1. The relative frequency of crossovers between gene pairs was found to decrease as the map is read from leu-6 to the right. The phenotype for arg-1 was found infrequently in recombinants. This fact, coupled with the observed decrease in crossover frequency dependent upon map position, suggested two alternative explanations to account for the observed results. (i) Zygote formation among these nocardiae is a consequence of partial, oriented chromosome transfer from N. erythropolis to N. canicruria and is similar to the mechanism of polar transfer reported in matings of Hfr by F(-)Escherichia coli. (ii) Being of diverse origin, the chromosomes of N. erythropolis and N. canicruria are not completely homologous, and decrease in crossover frequency dependent upon map position results from such chromosomal heterology. In this hypothesis, the infrequent occurrence of arg-1 among recombinants is considered to be an artifact resulting from the action of a previously unrecognized suppressor gene for arg-1 present in N. canicruria.

Linkage and segregation of unselected markers in matings of Nocardia erythropolis with Nocardia canicruria

The segregation of unselected genes expressing resistance or susceptibility to acriflavine, erythromycin, streptomycin, and tetracycline was analyzed in selected prototrophic recombinants resulting from matings of Nocardia erythropolis and N. canicruria. The organisms were shown to be functionally haploid and appeared to contain not more than one genome. It was postulated that all observed genes were present in a linear linkage group. The ordering of the genes in N. erythropolis was: tetB10 eryB9 his-3 purA1 acr-2 strA1 (respectively, resistance to tetracycline and erythromycin, deficiency for histidine and for purine, and resistance to acriflavine and streptomycin). The ordering of the genes in N. canicruria was: purB2 tetA9 eryA7 acr-11 strB2 (respectively, deficiency for purine, and resistance to tetracycline, erythromycin, acriflavine, and streptomycin). Excluding the genes for acriflavine resistance, acr-2 and acr-11, resistance loci in N. erythropolis were not allelic to and showed lateral displacement from genes controlling phenotypically similar resistance in N. canicruria. Evidence for some lack of homology between N. erythropolis and N. canicruria genomes was found. Recombination phenomena between the nocardial species was postulated to occur as a result of formation of a heterogenomic zygote in which new combinations were produced. Production of selectable, haploid recombinants was ascribed to subsequent haploidization of the zygote.

RECOMBINATION BETWEEN NOCARDIA ERYTHROPOLIS AND NOCARDIA CANICRURIA

Adams, James N. (University of South Dakota, Vermillion). Recombination between Nocardia erythropolis and Nocardia canicruria . J. Bacteriol. 88: 865–876. 1964.—Nutritionally complementary auxotrophic mutants derived from Nocardia erythropolis did not yield prototrophic recombinants when progeny from mixed cultures or newly mixed strains were inoculated onto minimal medium. Similarly, complementary auxotrophic mutants of N. canicruria did not produce prototrophic recombinants. When interspecific mating was attempted between complementary mutants of N. erythropolis and N. canicruria , prototrophic recombinants were recovered at frequencies dependent, in part, upon the auxotrophic strains used in the test crosses. Growth of the parental types in mixed cultures was necessary for the production of recombinant progeny. Direct selection for recombinants by inoculating the selective medium with a mixture of parental types without prior mixed growth did not result in the recovery of recombinants. Varying the medium upon which mixed growth occurred, or varying the ratio of N. erythropolis to N. canicruria cells used as inocula, did not greatly affect the recovery of recombinants. Heat-killing one or the other of the parental types prevented recombinant production. The lack of recovery of recombinants from crosses of homologously derived strains suggested that a mating factor controls recombination. The mating factor was not eliminated by acriflavine treatment. The recovery of a recombinant strain capable of forming recombinants with either N. erythropolis or N. canicruria , and the recovery of another strain which mated only with N. erythropolis in backcrosses, suggests that the mating factor may be of a multiple nature.