Increased rate of asporogenous mutations following treatment of Bacillus subtilis spores with ethyl methanesulfonate

Genes required for cytochrome c synthesis in Bacillus subtilis

Cytochromes of c-type contain covalently bound haem and in bacteria are located on the periplasmic side of the cytoplasmic membrane. More than eight different gene products have been identified as being specifically required for the synthesis of cytochromes c in Gram-negative bacteria. Corresponding genes are not found in the genome sequences of Gram-positive bacteria. Using two random mutagenesis approaches, we have searched for cytochrome c biogenesis genes in the Gram-positive bacterium Bacillus subtilis. Three genes, resB, resC and ccdA, were identified. CcdA has been found previously and is required for a late step in cytochrome c synthesis and also plays a role in spore synthesis. No function has previously been assigned for ResB and ResC but these predicted membrane proteins show sequence similarity to proteins required for cytochrome c synthesis in chloroplasts. Attempts to inactivate resB and resC in B. subtilis have indicated that these genes are essential for growth. We demonstrate that various nonsense mutations in resB or resC can block synthesis of cytochromes c with no effect on other types of cytochromes and little effect on sporulation and growth. The results strongly support the recent proposal that Gram-positive bacteria, cyanobacteria, epsilon-proteobacteria, and chloroplasts have a similar type of machinery for cytochrome c synthesis (System II), which is very different from those of most Gram-negative bacteria (System I) and mitochondria (System III).

Small, Acid-Soluble Spore Proteins of the alpha/beta Type Do Not Protect the DNA in Bacillus subtilis Spores against Base Alkylation

Ethyl methanesulfonate (EMS) killed wild-type Bacillus subtilis spores as rapidly as spores lacking small, acid-soluble proteins (SASP) of the alpha/beta type (alpha-beta- spores), and 20% of the survivors had obvious mutations. A recA mutation increased the EMS sensitivity of wild-type and alpha-beta- spores similarly but reduced their mutagenesis; EMS treatment of dormant spores also resulted in the induction of RecA synthesis during spore germination. EMS generated similar levels of alkylated bases in wild-type and alpha-beta- spore DNAs, in purified DNA, or in DNA saturated with alpha/beta-type SASP. Ethylene oxide (EtO) also generated similar levels of base alkylation in wild-type and alpha-beta- spore DNAs. These data indicate that EMS and EtO kill spores at least in part by DNA damage but that alpha/beta-type SASP, which protect DNA against many types of damage, do not protect spore DNA from base alkylation.

Identification and characterization of the ccdA gene, required for cytochrome c synthesis in Bacillus subtilis

The gram-positive, endospore-forming bacterium Bacillus subtilis contains several membrane-bound c-type cytochromes. We have isolated a mutant pleiotropically deficient in cytochromes c. The responsible mutation resides in a gene which we have named ccdA (cytochrome c defective). This gene is located at 173 degrees on the B. subtilis chromosome. The ccdA gene was found to be specifically required for synthesis of cytochromes of the c type. CcdA is a predicted 26-kDa integral membrane protein with no clear similarity to any known cytochrome c biogenesis protein but seems to be related to a part of Escherichia coli DipZ/DsbD. The ccdA gene is cotranscribed with two other genes. These genes encode a putative 13.5-kDa single-domain response regulator, similar to B. subtilis CheY and Spo0F, and a predicted 18-kDa hydrophobic protein with no similarity to any protein in databases, respectively. Inactivation of the three genes showed that only ccdA is required for cytochrome c synthesis. The results also demonstrated that cytochromes of the c type are not needed for growth of B. subtilis.

Transduction of certain genes by an autonomously replicating Bacillus thuringiensis phage

A derivative of Bacillus thuringiensis subsp. kurstaki HD1 (HD1-9) released transducing phage (TP21) from late exponential cultures. Three of seven markers tested were transduced into Bacillus cereus, but only two of these (cysC and trpB/F) were transduced at a frequency of more than 100 times the reversion rates. A limited transduction capacity was given further support in that few chromosomal markers were carried in the HD1-9 lysate, as demonstrated by Southern hybridization. Restriction fragments from the phage DNA and from total B. thuringiensis DNA hybridized to an insertion sequence (IS231-like) probe, which may provide a region of homology for transduction. All of the B. cereus transductants contained the phage as a 44-kb plasmid, and each could transduce both the cys and trp genes to other B. cereus auxotrophs, albeit at lower frequencies than those for the B. thuringiensis transducing phage. In some cases, especially for cys, the transduced gene was integrated into the chromosome of the recipient, whereas the trp gene in many cases appeared to be lost with curing of the 44-kb plasmid. In addition, some B. cereus transductants lost prototrophy but retained a 44-kb plasmid, consistent with the presence of TP21 helper phage. These phage may mediate the subsequent transduction from B. cereus phototrophs. TP21 replicates as a plasmid and, at least under the conditions studied, selectively transfers markers to B. cereus.

Plasmid transfer between strains of Bacillus thuringiensis infecting Galleria mellonella and Spodoptera littoralis

To determine the possibility of plasmid transfer occurring between strains of Bacillus thuringiensis in infected lepidopterous larvae, Galleria mellonella and Spodoptera littoralis were infected with two or more strains of B. thuringiensis and the resulting bacteria from the dead insects were examined for plasmid transfer. Transfer rates of plasmids coding for crystal production and tetracycline resistance were high, reaching levels similar to those obtained in laboratory broth cultures. Transfer was higher in G. mellonella than S. littoralis, probably due to the greater ability of B. thuringiensis to colonize the larvae. In broth cultures, B. thuringiensis was also able to transfer plasmids into sporeforming bacteria present in soil samples. The results suggest that plasmid transfer between strains of B. thuringiensis occurs in nature, resulting in the production of new combinations of delta-endotoxins within populations of the bacteria.

Isolation and characterization of outermost layer deficient mutant spores of Bacillus megaterium

Outermost layer deficient mutant spores of Bacillus megaterium ATCC 12872 were isolated by Urografin density gradient centrifugation after mutagenesis with ethyl methanesulfonate. Although the composition of the cortex peptidoglycan was the same as that of the parent spores, three major proteins (48, 36, and 22 K daltons) were missing, suggesting that these proteins are components of the outermost layer. All mutant spores were also found to have very hydrophobic surface by 'salt aggregation test,' which would facilitate selection of such mutants.

Transfer of chromosomal genes and plasmids in Bacillus thuringiensis

A low frequency of chromosomal gene transfer from Bacillus thuringiensis to Bacillus cereus was detected by cell mating, with a tryptophan marker being the most frequently transferred gene among four that were tested. The process was resistant to DNase and was not mediated by cell filtrates. Among several B. thuringiensis subspecies tested, transfer was best with a derivative of B. thuringiensis subsp. kurstaki HD1, which lost several plasmids. All of the B. cereus recombinants contained at least one plasmid from the donor B. thuringiensis; frequently, it was a plasmid that encoded a protoxin gene. In matings with B. thuringiensis subsp. kurstaki HD1, a 29-megadalton plasmid that contained a ca. 2.5-kilobase region of homology with the chromosome was always transferred. No detectable transfer of chromosomal genes was found in B. thuringiensis subsp. kurstaki HD1 strains lacking this plasmid, suggesting that there may be chromosome mobilization.

Regulation of protoxin synthesis in Bacillus thuringiensis

A derivative of Bacillus thuringiensis subsp. kurstaki (HD-1) formed parasporal inclusions at 25 degrees C, but not at 32 degrees C. This strain differed from the parent only in the loss of a 110-megadalton (Md) plasmid, but plasmid and chromosomal copies of protoxin genes were present in both strains. On the basis of temperature shift experiments, the sensitive period appeared to be during midexponential growth, long before the time of protoxin synthesis at 3 to 4 h after the end of exponential growth. The conditional phenotype could be transferred by cell mating to naturally acrystalliferous Bacillus cereus. In all such cases, a 29-Md protoxin -encoding plasmid was transferred, but this plasmid alone was barely sufficient for protoxin synthesis. Protoxin production increased to detectable levels, but well below those of the parental donor strain, by simultaneous transfer of a 44-Md protoxin -encoding plasmid. Transfer of a 5-Md plasmid with the two larger protoxin -coding plasmids resulted in a protoxin synthesis level approaching that of the donor strain. A role for some of the cryptic plasmids of kurstaki in parasporal body formation was implied. In contrast, a closely related B. thuringiensis strain, HD73 , produced crystals at both 25 and 32 degrees C even when the capacity was transferred on a 50-Md plasmid to B. cereus. The amount of protoxin produced in these B. cereus transcipients , however, was somewhat less than that produced in the parental strain HD73 , implying that catabolic differences, gene dosage, or the presence of a chromosomal gene (or a combination of these) may be necessary for maximum production. A regulatory component of the 29-Md plasmid appeared to be trans-acting and dominant since B. cereus transcipients containing the 29-Md plasmid from kurstaki and the 50-Md plasmid from HD73 produced more protoxin at 25 degrees C than at 30 degrees C. Similar results were obtained when protoxin synthetic capacity was transferred from B. thuringiensis subsp. israelensis to the conditional B. thuringiensis subsp. kurstaki strain.

Characterization of Bacillus subtilis mutants with a temperature-sensitive intracellular protease

A colony screening procedure was devised to detect Bacillus subtilis mutants containing temperature-sensitive trypsin-like intracellular protease activity. The enzyme was characterized as a non-sulfhydryl serine protease on the basis of inhibitor studies. It was also inhibited by D- or L-histidine but not by any other amino acid tested. The long-term survival at 45 degrees C of these mutants in a minimal salts medium was decreased, with rapid lysis occurring within 24 h. A D-histidine function in long-term survival and inhibition accounted for the presence of additional protease mutants among survivors of histidine auxotrophs selected for their ability to utilize D-histidine. In addition to being lysed when incubated at 45 degrees C under nongrowth conditions, all of the protease mutants had a decreased rate of protein turnover and produced spores deficient in a major low-molecular-weight spore coat polypeptide. The morphology of the undercoat layers was altered, but there was no effect on spore heat resistance or on germination. The missing spore coat polypeptide appeared to be processed from a larger precursor by cleavage to produce N-terminal histidine. A defect in this protease could account for the lack of processing and thus the absence of this polypeptide in spore coats.

Reconstitution of succinate dehydrogenase in Bacillus subtilis by protoplast fusion

Bacillus subtilis succinate dehydrogenase (SDH) is composed of two unequal subunits designated Fp (Mr, 65,000) and Ip (Mr. 28,000). The enzyme is structurally and functionally complexed to cytochrome b 558 (Mr, 19,000) in the membrane. A total of 21 B. subtilis SDH-negative mutants were isolated. The mutants fall into five phenotypic classes with respect to the presence and localization of the subunits of the SDH-cytochrome b558 complex. One class contains mutants with an inactive membrane-bound complex. Membrane-bound enzymatically active SDH could be reconstituted in fused protoplasts of selected pairs of SDH-negative mutants. Most likely reconstitution is due to the assembly of preformed subunits in the fused cells. On the basis of the reconstitution data, the mutants tested could be divided into three complementation groups. The combined data of the present and previous work indicate that the complementation groups correspond to the structural genes for the three subunits of the membrane-bound SDH-cytochrome b558 complex. A total of 31 SDH-negative mutants of B. subtilis have now been characterized. The respective mutations all map in the citF locus at 255 degrees on the B. subtilis chromosomal map. In the present paper, we have revised the nomenclature for the genetics of SDH in B. subtilis. All mutations which give an SDH-negative phenotype will be called sdh followed by an isolation number. The designation citF will be omitted, and the citF locus will be divided into three genes: sdhA, sdhB, and sdhC. Mutations in sdhA affect cytochrome b558, mutations in sdhB affect Fp, and mutations in sdhC affect Ip.

Characterization of a thermosensitive sporulation mutant of Bacillus subtilis affected in the structural gene of an intracellular protease

A thermosensitive sporulation mutant (ts-15) of Bacillus subtilis has been isolated. This mutant when grown at the restrictive temperature (42 degrees C) is unable to sporulate, shows no intracellular protease activity and no protein turnover. These three traits were recovered in two revertants (ts-15R1 and ts-15R2) and were also transmitted together by transformation into the wild type. Immunological studies have shown that when ts-15 is grown at 42 degrees C it synthesizes a 'cryptic' protein with apparently the same antigenic properties as the wild type or as ts-15 mutant grown at the permissive temperature (30 degrees C). The intracellular proteases from the wild type and from ts-15 grown at 30 degrees C and 42 degrees C were completely purified and their properties were studied with respect to their molecular weights, substrate specificity, inhibition pattern, heat inactivation and antigenicity. The molecular weight of the enzyme from the wild type or ts-15 grown at 30 degrees C was 64000--65000 in the absence of sodium dodecylsulfate and 31000--32000 in the presence of sodium dodecylsulfate. It was assumed therefore that the active enzyme is formed from two similar subunits. However, the intracellular protease from ts-15 grown at 42 degrees C showed the same molecular weight of 32000--34000 in the presence or in the absence of sodium dodecylsulfate. On the basis of this experiment and others described in the paper we concluded that the mutation in ts-15 is most likely a point mutation in a structural gene of an intracellular protease and results in an inability to assemble the two subunits into an active form.

Properties of Bacillus cereus temperature-sensitive mutants altered in spore coat formation

Three conditional Bacillus cereus mutants altered in the assembly or formation of spore coat layers were analyzed. They all grew as well as the wild type in an enriched or minimal medium but produced lysozyme and octanol-sensitive spores at the nonpermissive temperature (35 to 38 degrees C). The spores also germinated slowly when produced at 35 degrees C. Temperature-shift experiments indicated that the defective protein or regulatory signal is expressed at the time of formation of the outer spore coat layers. Revertants regained all wild-type spore properties at frequencies consistent with initial point mutations. Spore coat defects were evident in thin sections and freeze-etch micrographs of mutant spores produced at 35 degrees C. In addition, one mutant contained an extra surface deposit, perhaps unprocessed spore coat precursor protein. A prevalent band of about 65,000 daltons (the same size as the presumptive precursor) was present in spore coat extracts of this mutant and may be incorrectly processed to mature spore coat polypeptides. Another class of mutants was defective in the late uptake of half-cystine residues into spore coats. Such a defect could lead to improper formation of the outer spore coat layers.

Characterization of a Bacillus cereus protease mutant defective in an early stage of spore germination

Temperature-sensitive sporulation mutants of Bacillus cereus were screened for intracellular protease activity that was more heat labile than that of the parental strain. One mutant grew as well as the wild type at 30 and 37 degrees C but sporulated poorly at 37 degrees C in an enriched or minimal medium. These spores germinated very slowly in response to alanine plus adenosine or calcium dipicolinate. During germination, spores produced by the mutant rapidly became heat sensitive, but released dipicolonic acid and mucopeptide fragments more slowly than the wild type and decreased only partially in density while remaining phase white (semirefractile). In freeze-etch electron micrographs, the mature spores were deficient in the outer cross-patched coat layer. During germination, the spore coat changes associated with wild-type germination occurred very slowly in this mutant. Although the original mutant was also a pyrimidine auxotroph, reversion to prototrophy did not alter any of the phenotypic properties discussed. Selection of revertants that germinated rapidly or sporulated well at 37 degrees C, however, resulted in restoratin of all wild-type properties (exclusive of the pyrimidine requirement) including heat-stable protease activity. The reversion frequency was consistent with an initial point mutation, indicating that a protease alteration resulted in production of spores defective in a very early stage of germination.

Bacteriophage conversion of spore-negative mutants to spore-positive in Bacillus pumilus

A pseudolysogenic phage, PMB1, was isolated from soil on the basis of its ability to increase the sporulation frequency of the oligosporogenic Bacillus pumilus strain NRS 576 (sporulation frequency, less than 1%). Several spore-negative mutants (sporulation frequency, less than 10-8) derived from strain NRS 576, which were converted to spore positive by infection with PMB1, were subsequently identified. PMB1 repeatedly grown on a given spore-negative mutant (e.g., GW2) converted GW2 cells to spore positive. Each plaque-forming unit initiated the conversion of a spore-positive clone in semisolid agar overlays. GW2 cells remained spore positive as long as they maintained PMB1. Return of PMB1-converted cells to the orginal spore-negative phenotype correlated with loss of PMB1. In liquid media, PMB1 infection increased the sporulation frequency of mutant GW2 over 106-fold. More than half of the spore-negative mutants we isolated from strain NRS 576 were converted to spore positive by PMB1 infection. PMB1-induced spores of the spore-negative mutant GW2 were somewhat more heat sensitive than uninfected or PMB1-infected spores of the spore positive parent of GW2. PMB1-induced spores of GW2 do not differ from wild-type spores in morphology by phase-contrast microscopy, dipicolinic acid content, or rate of sedimentation through Renografin gradients.

Isolation of acetyl esterase mutants of Bacillus subtilis 168

Five mutants of Bacillus subtilis 168 defective in an intracellular esterase activity were identified. By polyacrylamide gel electrophoresis, four of the mutants were shown to lack esterase B activity, and the fifth lacked esterase A activity. All of the back-crossed esterase mutants were able to sporulate at wild-type frequency and produce exoprotease(s) and antibiotic(s). No difference in motility could be attributed to the esterase mutation. PBS1 transduction analysis showed all the esterase B mutations to be linked to the hisA marker.

The sporulation system of Bacillus subtilis as the basis of a multi-gene mutagen screening test

The sporulation system of B. subtilis provides the basis of a simple and unique test for the detection of forward mutations in any of several hundreds genes in 28--45 separate operons scattered throughout the chromosome. Non-sporulating or oligosporogenous mutant colonies are easily identified by their lack of a brown pigment normally present in spore-forming colonies. N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG), ethyl methanesulfonate, acridine orange, acriflavin, nitrous acid, and UV irradiation are already known to produce sporulation mutants. This paper reports the dose dependence of sporulation mutant induction by 2-nitrosofluorene, ICR-191, nitrogen mustard, ethidium bromide and MNNG; mutagenesis is also demonstrated for aflatoxin B1 and 4-nitroquinoline-N-oxide. A mammalian liver enzyme metabolizing system was necessary for activation of aflatoxin B1. Auramine-O and 4-nitro-o-phenylenediamine failed to give a significant mutagenic effect under the conditions employed. The wide variety of mutagen classes detected indicates the general applicability of the test. This test, based on many genes throughout the chromosome, may prove less apt to exclude rare mutagenic "hot-spots" than systems based on the detection of mutations in a restricted region of the chromosome.

Properties of Bacillus cereus spore coat mutants

Two classes of spore mutants have been selected in Bacillus cereus T, those producing lysozyme-sensitive spores, and those producing spores dependent upon lysozyme for germination. One mutant from each class was studied in detail and found to have defective packing of the spore coat layers. The major spore coat poplypeptide appeared to be altered on the basis of gel electrophoretic profiles and/or peptide maps of half-syctine-containing peptides. The spores of the mutants of both classes were sensitive to lysozyme and failed to respond to the germinants L-alanine plus adenosine. The spores were also more sensitive to octanol than the parental strain, but contained the same amount of dipicolinic acid and were equally heat resistant. The reversion frequencies in both cases were consistent with an initial point mutation, suggesting that an alteration in the major coat polypeptide accounted for the phenotypic properties studied.