Partial exclusion of the Nocardia erythropolis chromosome in nocardial recombinants

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.

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.

Heterogenomic recombinants from compatible nocardiae

Recombinants obtained from matings of Nocardia erythropolis x N. canicruria were tested for their genetic stability by comparing phenotypes from direct selection with the same population after unselected growth. Contraselective loci were employed in various combinations in order that all of the mapped characters might be subjected to unselected analysis. Some recombinant class types appeared as stable haploids, whereas others behaved as heterozygous diploids, segregating out new phenotypes. All regions of the parental genomes were found to be involved in segregation, implying that the entire mapped region can become merozygotic under standard mating conditions. On the basis of segregating phenotypes, the genetic potentials of these compatible nocardiae were ascertained as follows: the formation of a diploid with subsequent segregation of parental or haploid recombinant genomes or both; persistence of the diploid through many generations; continuing reassortment of genetic information by multiple matings between parental or recombinant organisms; and, very probably, second-round recombinations within the diploid. A considerable difference in the nuclear division time between the parental organisms was postulated to have significant effects on the nature of the unselected segregants.

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.