Inhibitory protein controls the reversion of protoplasts and L forms of Bacillus subtilis to the walled state

Discovery of a Novel Inner Membrane-Associated Bacterial Structure Related to the Flagellar Type III Secretion System

The bacterial flagellar type III secretion system (fT3SS) is a suite of membrane-embedded and cytoplasmic proteins responsible for building the flagellar motility machinery. Homologous nonflagellar (NF-T3SS) proteins form the injectisome machinery that bacteria use to deliver effector proteins into eukaryotic cells, and other family members were recently reported to be involved in the formation of membrane nanotubes. Here, we describe a novel, evolutionarily widespread, hat-shaped structure embedded in the inner membranes of bacteria, of yet-unidentified function, that is present in species containing fT3SS. Mutant analysis suggests a relationship between this novel structure and the fT3SS, but not the NF-T3SS. While the function of this novel structure remains unknown, we hypothesize that either some of the fT3SS proteins assemble within the hat-like structure, perhaps including the fT3SS core complex, or that fT3SS components regulate other proteins that form part of this novel structure.

IMPORTANCE The type III secretion system (T3SS) is a fascinating suite of proteins involved in building diverse macromolecular systems, including the bacterial flagellar motility machine, the injectisome machinery that bacteria use to inject effector proteins into host cells, and probably membrane nanotubes which connect bacterial cells. Here, we accidentally discovered a novel inner membrane-associated complex related to the flagellar T3SS. Examining our lab database, which is comprised of more than 40,000 cryo-tomograms of dozens of species, we discovered that this novel structure is both ubiquitous and ancient, being present in highly divergent classes of bacteria. Discovering a novel, widespread structure related to what are among the best-studied molecular machines in bacteria will open new venues for research aiming at understanding the function and evolution of T3SS proteins.

Loss of the Bacterial Flagellar Motor Switch Complex upon Cell Lysis

The bacterial flagellar motor is a complex macromolecular machine whose function and self-assembly present a fascinating puzzle for structural biologists. Here, we report that in diverse bacterial species, cell lysis leads to loss of the cytoplasmic switch complex and associated ATPase before other components of the motor. This loss may be prevented by the formation of a cytoplasmic vesicle around the complex. These observations suggest a relatively loose association of the switch complex with the rest of the flagellar machinery. IMPORTANCE We show in eight different bacterial species (belonging to different phyla) that the flagellar motor loses its cytoplasmic switch complex upon cell lysis, while the rest of the flagellum remains attached to the cell body. This suggests an evolutionary conserved weak interaction between the switch complex and the rest of the flagellum which is important to understand how the motor evolved. In addition, this information is crucial for mimicking such nanomachines in the laboratory.

Expression and maintenance of plasmid resistance in regenerating protoplasts of Bacillus subtilis

Expression of plasmid-encoded resistances in regenerating protoplasts of Bacillus subtilis occurs only after wall synthesis has been resumed. This is observed for protoplasts obtained from cells already containing plasmids (pC194, pT127, pK545) or for plasmid-bearing cells coming from a PEG-mediated transformation. Recovery of expression needs a 2-h incubation of protoplasts, previously washed to get rid of their lysozyme content, in rich hypertonic medium (SMMP). A longer incubation (24-h) results in the obtention of regenerants; however, most of them have lost their resistant phenotype in contrast to those obtained from the usual solid regeneration plates. This finding suggests either a high curing effect or some kind of gene inactivation phenomenon. Discussion is focused on the critical points that have to be considered when polyethylenglycol-mediated transformation of protoplasts is applied to recombinant DNA technology.

Genetic analysis of Bacillus stearothermophilus by protoplast fusion

Efficient and reliable protoplasting, regeneration, and fusion techniques were established for the prototrophic strain Bacillus stearothermophilus NUB36. Auxotrophic mutants were isolated, and protoplast fusion was used to construct isogenic mutant strains and for chromosomal mapping. Markers were mapped using two-, three-, and four-factor crosses. The order of the markers was hom-1-thr-1-his-1-(gly-1 or gly-2)-pur-1-pur-2. These markers may be analogous to hom, thrA, hisA, glyC, and purA markers on the Bacillus subtilis chromosome. No analogous pur-1 marker has been reported in B. subtilis. The relative order of three of the markers (hom-1-thr-1-gly-1) was independently confirmed by transduction.

Transformation of Clostridium perfringens

Clostridium perfringens 11268 CDR (Rifr Tcs), the strain transformed in our experiments, was generated by curing a spontaneous, rifampicin-resistant mutant of C. perfringens 11268 (Rifr Tcr). High-temperature growth yielded tetracycline-sensitive, rifampicin-resistant cells which no longer contained pCW3, a 42.8-kilobase plasmid. The tetracycline-sensitive, rod-shaped cell was then converted to an L-phase variant by growth in the presence of penicillin G (10 micrograms/ml) and 0.4 M sucrose. After several passages, the antibiotic was removed from the medium, and cells continued to grow as L-phase variants. Another large plasmid, pJU124 (38.8 kilobases), which confers tetracycline resistance, was used for transformation. Transformation of L-phase variants of C. perfringens 11268 CDR (Rifr Tcs) was mediated by polyethylene glycol. Transformation frequency is a nonlinear function of DNA concentration. Restriction analysis showed that the plasmid isolated from the transformants was identical to that supplied. Stable L-phase variants do not revert to rod-shaped cells, but autoplasts can be both transformed and reverted.

Protoplast transformation of glutamate-producing bacteria with plasmid DNA

A method for polyethylene glycol-induced protoplast transformation of glutamate-producing bacteria with plasmid DNA was established. Protoplasts were prepared from cells grown in the presence of penicillin by treatment with lysozyme in a hypertonic medium. The concentration of penicillin during growth affected the efficiency of formation, regeneration, and polyethylene glycol-induced DNA uptake of protoplasts. Regeneration of protoplasts was accomplished on a hypertonic agar medium containing sodium succinate and yeast extract. The spectinomycin and streptomycin resistance plasmid pCG4, originally from Corynebacterium glutamicum T250, could transform various glutamate-producing bacteria such as C. glutamicum, Corynebacterium herculis, Brevibacterium flavum, and Microbacterium ammoniaphilum. The plasmid was structurally unchanged and stably maintained in new hosts. The transformation frequency of most competent protoplasts with pCG4 DNA isolated from primary transformants was high (ca. 10(6) transformants per microgram of covalently closed circular DNA) but was still two orders of magnitude below the frequency of transfection with modified DNA of the bacteriophage phi CGI. The difference was ascribed to the involvement of regeneration in transformation.

Induction of L-phase variant from protoplast of Staphylococcus aureus

Protoplasts of Staphylococcus aureus 209P and Cowan 1 were induced by treatment with lysostaphin. These protoplasts were sensitive to detergent, a low concentration of sodium chloride and low temperature. Almost all protoplast cells spread on CLYS agar medium (casein hydrolysate, yeast extract, Na-lactate, and NaCl) formed typical L-form colonies. Horse serum (0.25%) and Mg2+ (100 mM) are essential factors for formation of the L-form colonies of 209P. In the case of Cowan 1, Mg2+ was not required. The active factor(s) in horse serum was heat-resistant and protein in nature.

Computer-assisted chromosome mapping by protoplast fusion in Staphylococcus aureus

Protoplasts of genetically marked derivatives of Staphylococcus aureus NCTC 8325 were fused with polyethylene glycol and regenerated without selection. Recombinants possessing one specific resistance marker from each parent were selected from the regenerated population and scored for seven or eight unselected markers. The results of these 9- and 10-factor crosses were entered directly into a programmed microcomputer from prescored replica plates. The data then were condensed into an array of phenotypes, together with the frequency with which each occurred. Further analyses by computer included the calculation of coinheritance frequencies for all possible pairs of markers; after entering a proposed order for the markers being analyzed, the minimum number of crossover events required to generate each phenotypic class was calculated. The linkage relationships of markers, based on the protoplast fusion data, were entirely consistent with the linkage relationships of markers already known to exist within each of the three linkage groups previously defined by transformation. The fusion data defined an arrangement of the three linkage groups into a circular chromosome map and predicted the approximate location of four previously unmapped markers (tet-3490, fus-149, purC193::Tn551, and omega [Chr::Tn551]42) on this map.

Biparental products of bacterial protoplast fusion showing unequal parental chromosome expression

Efficiently regenerated single colonies from mixed multiply auxotrophic Bacillus subtilis protoplasts, fused with polyethylene glycol, reveal colonies carrying each of the parent types (biparentals) and recombinant colonies. The latter appear in high yields (up to 1% of certain recombinant classes); the yield of biparentals may be as large as 10%, in the range of the indicated physical fusion events. Many of the biparentals are diploids although, contrary to expectation, they are not complementing prototrophs, but show precisely the phenotype of one (either one) of the parent strains. Extensive pedigree analysis and subcloning of diploid lines show that they can propagate with varying stability on the appropriate parental selective medium to reproduce diploid progeny, parental segregants, and late-appearing recombinants, including some prototrophs. Thus, the principal product of intertype protoplast fusion is a diploid carrying two chromosomes, only one of which is expressed in each particular clone. The extinction of one parental genome is especially well demonstrated when it includes suppression of a normally dominant antibiotic sensitivity marker. In transformation experiments, DNA made from a selected diploid clone was able to transfer several of the unexpressed genes. The structural or topological character of DNA associated with the chromosome extinction remains unexplained.

Parameters governing bacterial regeneration and genetic recombination after fusion of Bacillus subtilis protoplasts

Bacterial protoplast fusion, induced by polyethylene glycol, has been made more regular and convenient by further specification and improvement of various steps in the previously used procedure. These have made it possible to obtain regularly 100% regeneration of Bacillus subtilis cells from protoplasts before treatment with polyethylene glycol and yields of 10 to 75% from polyethylene glycol-treated protoplasts. Genetic recombination frequencies do not increase correspondingly. Also, when regeneration is reduced by various experimental conditions, recombination does not decrease in proportion. It is concluded that regeneration of recombinant-forming cells is independently determined and not closely related to the average regeneration for the population. Kinetic studies with varying individual parental or total protoplast concentrations strongly indicate that protoplast collision and contact is not the limiting factor determining the number of genetic recombinants obtained. Recombination approximates a linear, rather than quadratic, function of the total or of the majority protoplast population present, from which it is concluded that fusion events are always adequate to produce substantially more potential recombinants than are registered. The strong effect of the majority/minority ratio upon the number of minority cells that become recombinant is independent of which parent is in excess. This shows in a direct and physiological way that both parents are equivalent partners in their genetic contributions.

Effects of magnesium, calcium, and serum on reversion of stable L-forms

The L-form of Agromyces ramosus was stable in the absence of penicillin when transferred on heart infusion agar containing NaCl and serum. It reverted to its bacterial form, however, when magnesium replaced the serum in this medium. On a dilute medium containing NaCl but lacking serum, the L-form died out unless calcium, magnesium, or serum was added. It grew as the L-form in the presence of calcium of serum but reverted to the bacterial form in the presence of magnesium. Reversion also occurred when magnesium was added to the dilute medium containing serum. Calcium interfered with or prevented the magnesium-induced reversion. The revertant bacterial form resulting from these studies was not NaCl sensitive, as was the case of the bacterial revertant of this organism produced in soil (A. H. Horwitz and L. E. Casida, Jr., Can. J. Microbiol, 24:50--55, 1978).

Inhibition of peptidoglycan biosynthesis at a postcytoplasmic reaction in a stable L-phase variant of Streptococcus faecium

Cultures of a stable L-phase variant of Streptococcus faecium F24 produced and retained peptidoglycan precursors intracellularly over the entire growth cycle in a chemically defined medium. The identity of the most abundant precursor, UDP N-acetylmuramyl-L-alanyl-D-glutamyl-L-lysyl-D-alanyl-D-alanine (UDP-MurNAc-pentapeptide), was confirmed by demonstrating in vitro the presence of enzymes required for the cytoplasmic stage of peptidoglycan biosynthesis. The initial membrane-bound reaction in peptidoglycan biosynthesis involving phospho-MurNAc-pentapeptide translocase and undecaprenyl-phosphate membrane carrier was catalyzed by protoplast membrane preparations but not by L-phase membrane preparations. However, both protoplast and L-phase membranes incorporated radioactivity from dTDP-L-[14C]rhamnose, the presumed precursor to a non-peptidoglycan cell surface component, into high-molecular-weight material. dTDP-L-rhamnose did not accumulate in growing cultures but was synthesized from D-glucose-1-phosphate and dTTP by cell-free extracts of the streptococcus and L-phase variant. Neither rhamnose- nor muramic acid-containing compounds were detected in culture fluids. It is suggested that continued inhibition of cell wall biosynthesis in this stable L-phase variant is the result of a defect expressed at the membrane stage of peptidoglycan biosynthesis specifically involving the translocation step.