Complementation and genetic inactivation: two alternative mechanisms leading to prototrophy in diploid bacterial clones

Tn10 mutagenesis in Azotobacter vinelandii

The tetracycline-resistant transposon Tn10 and its "high-hopper" derivative Tn10HH104 were introduced into the Azotobacter vinelandii genome using suicide conjugative plasmids derived from pRK2013. Several types of mutants induced by either of these elements are described. Nif- mutants (deficient in nitrogen fixation) were easily isolated, whereas the isolation of other mutant types (auxotrophs, sugar non-users) required special selection conditions. The characterization of the mutations as transposon insertions was often complicated and sometimes required a combination of genetic and physical tests. A common source of complication, the existence of double inserts, was found among the mutants induced by Tn10HH104 but not among those induced by Tn10. Both the high-hopper and the wild-type element proved to undergo secondary transpositions, albeit at different frequencies. Another type of complication, the existence of heterozygotes, occurred because of the high level of redundancy of the A. vinelandii genome.

Noncomplementing diploidy resulting from spontaneous zygogenesis in Escherichia coli

With the aim of understanding sexual reproduction and phenotypic expression, a novel type of mating recently discovered in Escherichia coli was investigated. Termed spontaneous zygogenesis (or Z-mating), it differs from F-mediated conjugation. Its products proved phenotypically unstable, losing part of the phenotype for which they were selected. Inactivation of a parental chromosome in the zygote is strongly suggested by fluctuation tests, respreading experiments, analysis of reisolates, and segregation of non-viable cells detected by epifluorescence staining. Some phenotypically haploid subclones were interpreted as stable noncomplementing diploids carrying an inactivated co-replicating chromosome. Pedigree analysis indicated that the genetic composition of such cells consisted of parental genomes or one parental plus a recombinant genome. Inactivation of a chromosome carrying a prophage resulted in the disappearance of both the ability to produce phage particles and the immunity to superinfection. Phage production signalled transient reactivation of such a chromosome and constituted a sensitive test for stable noncomplementing diploidy. Chromosome inactivation thus appears to be a spontaneous event in bacteria.

Protoplast fusion: A tool for intergeneric gene transfer in bacteria

Protoplasts can be isolated from bacterial cells by digestion of the cell wall with the help of lysozyme in presence of osmotic stabilizers. Fusion of protoplasts can be induced by chemical fusogens like polyethylene glycol. The electrofusion technique has been reported in bacteria in which the fusion frequency is much higher than that obtained by PEG induced protoplast fusion. This technology allows recombination to take place not only between related species but also between unrelated genera and is of great potential in the breeding and improvement of industrial strains. This review includes the information and developments on the protoplast fusion in bacteria with special reference to genetic recombination by protoplast fusion between phylogenetically unrelated bacteria.

The spo0A gene is implicated in the maintenance of non-complementing diploids in Bacillus subtilis

Bacillus subtilis can exist in a diploid state in which two genetically distinct chromosomes co-exist in the same cell and yet only one of them is expressed, thereby determining the phenotype. Such cells are called non-complementing diploids (Ncds). In this study, two types of experiments are reported which indicate that a previously known pleiotropic gene, spo0A, plays a role in the maintaining the diploid state, as follows. (i) When protoplasts of two Spo0A mutant strains were fused, the resulting products continued to segregate cells of both parental phenotypes for many more divisions than had been reported previously. (ii) When a stable Ncd (an Ncd in which the unexpressed markers are not spontaneously activated at a detectable level) harbouring a chloramphenicol acetyltransferase gene on the silent chromosome was transformed with spo0A null alleles the transformants often expressed chloramphenicol acetyltransferase activity. Together these results indicate that the spo0A gene is involved in maintenance of the diploid state in both unstable and stable Ncds.

Genetic analysis of fusion recombinants in Bacillus subtilis: function of the recE gene

Bacillus subtilis protoplast fusion allows the study of the genetic recombination of an entire procaryotic genome. Protoplasts from bacterial strains marked genetically by chromosomal mutations were fused using polyethylene glycol and the regenerated cells analyzed. Recombinants represent 19.3% of heterozygotic cells; they are haploids. Individual characterization of clones show a unique particular phenotype in each colony suggesting that recombination takes place immediately after fusion, probably before the first cellular division. Recombination occurs in the whole chromosome; in one-third of the cases both reciprocal recombinants could be shown in the colony. The genetic interval that includes the chromosome replication origin shows the highest recombination level. Our results suggest that the RecE protein accounts for most of the fused protoplast recombination; however, some "replication origin-specific" recombination events were independent of the recE gene product.

Further studies on recombination in diploid clones from Bacillus subtilis protoplast fusion

Diploid prototrophs were obtained from protoplast fusion of Bacillus subtilis strains. They are unstable but upon further cultivation they stabilize retaining diploidy but are genetically inactive. It has been suggested that recombination between the parental chromosomes is involved in the production of stable prototrophs and recombinants. In this work the occurrence of this recombination was searched for by determining genetic linkages in transformation experiments. In prototrophs two alleles: hisH2 and trpE8 carried originally on each parental chromosome, were shown to be 48% co-transformable in a stable clone whereas they were only cotransformed in 10% of the unstable colonies. For Trp- recombinants (the most frequent type of a Leu- Met- Thr- x Ade- Ura- Trp- fusion pair) lysed protoplasts were used as donor DNA for the transformations. High values of co-transfer for Ura+ Met+ were obtained. These results confirm the occurrence of recombination in stable diploid clones, prototrophs or recombinants.