Mechanisms of tetracycline and rifampicin resistance inBacteroides fragilis mutants obtained by ionizing radiation

Summary Based on a dose-survival curve, a radiation dose of 3.99 C/kg was used to induce antibiotic-resistant mutants inBacteroides fragilis. Escherichia coli B/r membrane fragments were employed as a reducing agent. Antibiotic-resistant mutants ofB. fragilis were utilized to study the mechanism by which these organisms become resistant to selected chemotherapeutic agents. Decreased accumulation of tetracycline by resistant mutants ofB. fragilis suggests that the resistance to this antibiotic is associated with the outer membrane permeability. There is a marked difference in the inhibitory action of rifampicin on RNA polymerase activity in rifampicin-sensitive and-resistant strains ofB. fragilis. This enzyme is, therefore, the likely target for inhibition of bacterial growth in this anaerobe by rifampicin.     

Resistance ofSpodoptera frugiperda [Lep.: Noctuidae] to a nuclear polyhedrosis virus in the field and laboratory

Median lethal doses (LD₅₀s) of nuclear polyhedrosis virus (NPV) were determined in neonatal offspring ofSpodoptera frugiperda (J. E. Smith) (Sf) larvae captured in southeastern Louisiana in 1981, 1982, and 1984. These LD₅₀s ranged from 1.8 to 16.3 polyhedral inclusion bodies (PIB)/insect. The LD₅₀s significantly (P<0.05) increased during the season of 1982 but had no pattern in 1981 or 1984. However, the Sf populations increased in heterogeneity of response to the NPV during all 3 years. The LD₅₀ increased from 4.1 to 18.7 PIB/insect in a Sf laboratory colony exposed to the NPV LD₈₀ for 7 generations, whereas in a control colony not exposed to NPV the LD₅₀ was 5.9 PIB/insect after 7 generations.     

Insertional inactivation of a gene which controls expression of vancomycin resistance on plasmid pHKK100

Expression of inducible high level vancomycin resistance (Vmr) in enterococci appears to require other plasmid-encoded genes in addition to the previously described structural genes vanA and vanH. Tn917 mutagenesis was used to identify such a region in the Vmr plasmid pHKK100. Insertional inactivation of a 693-bp open reading frame upstream from vanH resulted in complete loss of Vmr. This putative 26,642-Da protein has been designated VanR.     

Construction of a new universal vector for insertional mutagenesis by homologous recombination

We describe here the construction of a vector (pSSC-9) which can be used for the insertional mutagenesis of any gene for which genomic sequences have been cloned. This vector contains a neomycin-resistance-encoding gene (neo^R) which is driven by a modified thymidine kinase (tk) promoter for positive selection. Flanking neo^R are two tk genes driven by their own promoters for negative selection of nonhomologous insertions. The neo^R and tk cassettes are separated by four unique cloning sites on the right-hand side of the neo^R cassette and three unique sites on the left-hand side. The vector also includes two SfiI sites, one on each side of the tk cassettes, for the excision of the cloned genomic DNA fragments along with the selectable markers. Electroporation of pSSC-9 into mouse embryonic stem (ES) cells and cultured diploid mouse adrenal Y-1 cells conferred resistance to G418 and sensitivity to ganciclovir in both cell lines. These results illustrate the expression of the positive and negative selectable markers in two different cell lines and thus suggest that the vector could be used in ES cells, as well as in cultured somatic cells.     

Reduced neuronal sensitivity to dieldrin and picrotoxinin in a cyclodiene-resistant strain of Drosophila melanogaster (Meigen).

Toxicological and neurophysiological studies were performed to characterize the resistance mechanism in a cyclodiene-resistant strain of Drosophila melanogaster (Maryland strain). Dieldrin had an LC50 of 0.058 ppm against the larvae of susceptible D. melanogaster (Oregon-R wild type) when formulated in the rearing media. The LC50 of the resistant Maryland strain was 10.8 ppm, giving a resistance ratio (LC50-Maryland/LC50-susceptible) of 186-fold. Suction electrode recordings were made from peripheral nerves of the larval central nervous system to test whether reduced nerve sensitivity played any role in the observed resistance. In susceptible preparations (n = 5), inhibition of nerve firing by 1 mM gamma-aminobutyric acid (GABA) was effectively antagonized within 3-10 min by 10 microM dieldrin. In contrast, 30 min incubations with 10 microM dieldrin had no effect on preparations from cyclodiene-resistant individuals (n = 5). Similarly, 10 microM picrotoxinin blocked GABA-dependent inhibition in susceptible nerve preparations (n = 3). In recordings from resistant insects (n = 4), picrotoxinin displayed either weak antagonism of GABA or hyperexcitation indistinguishable from susceptible preparations. These results demonstrate that cyclodiene resistance in the Maryland strain of D. melanogaster 1) is expressed in immature stages, 2) is present at the level of the nerve, and 3) extends to picrotoxinin, albeit at a reduced level compared with dieldrin. The possible role of an altered GABA receptor in this resistance is discussed.     

Organisation and functions of the actV A region of the actinorhodin biosynthetic gene cluster of Streptomyces coelicolor

Summary Sequence analysis of the actVA region of the actinorhodin biosynthetic gene cluster of Streptomyces coelicolor revealed a succession of six open reading frames (ORFs), all running in the same direction and extending over 5.32 kb. The protein product of actVA-ORF1 strongly resembles that of another gene, elsewhere in the act cluster (actII-ORF2), which codes for a trans-membrane protein previously implicated in actinorhodin export from the mycelium. This suggests that the two gene products may co-operate in actinorhodin export, perhaps being sufficient for self-protection of the organism against suicide. At least four of the other five ORFs are implicated in the control of the C-6 and C-8 ring-hydroxylation reactions, lacking in actVA mutants, that occur at middle to late stages in the actinorhodin biosynthetic pathway. This conclusion was reached by genetic mapping of actVA mutants to actVA-ORF3 and-ORF5 (and perhaps -ORF4), and by the finding of strong resemblances between the protein products of actVA-ORF2 and -ORF6 and the products of genes of the oxytetracycline or tetracenomycin gene clusters that have been implicated in ring-hydroxylation reactions in the biosynthesis of these other aromatic polyketide antibiotics.     

Recombination at the Rp1 locus of maize.

Summary The Rp1 locus of maize determines resistance to races of the maize rust fungus (Puccinia sorghi). Restriction fragment length polymorphism markers that closely flank Rp1 were mapped and used to study the genetic fine structure and role of recombination in the instability of this locus. Susceptible progeny, lacking the resistance of either parent, were obtained from test cross progeny of several Rp1 heterozygotes. These susceptible progeny usually had non-parental genotypes at flanking marker loci, thereby verifying their recombinational origin. Seven of eight Rp1 alleles (or genes) studied were clustered within about 0.2 map units of each other. Rpl ^G, however, mapped from 1–3 map units distal to other Rp1 alleles. Rp5 also mapped distally to most Rp1 alleles. Other aspects of recombination at Rp1 suggested that some alleles carry duplicated sequences, that mispairing can occur, and that unequal crossing-over may be a common phenomenon in this region; susceptible progeny from an Rp1 ^A homozygote had recombinant flanking marker genotypes, and susceptible progeny from an Rp1 ^DlRp1 ^F heterozygote showed both possible nonparental flanking marker genotypes.     

Mutation proximal to the tRNA binding region of the Nicotiana plastid 16S rRNA confers resistance to spectinomycin.

Summary Nicotiana tabacum lines carrying maternally inherited resistance to spectinomycin were obtained by selection for green callus in cultures bleached by spectinomycin. Two levels of resistance was found. SPC1 and SPC2 seedlings are resistant to high levels (500 g/ml), SPC23 seedlings are resistant to low levels (50 g/ml) of spectinomycin. Lines SPC2 and SPC23 are derivatives of the SR1 streptomycin-resistant plastome mutant. Spectinomycin resistance is due to mutations in the plastid 16S ribosomal RNA: SPC1, an A to C change at position 1138; SPC2, a C to U change at position 1139; SPC23, a G to A change at position 1333. Mutations similar to those in the SPC1 and SPC2 lines have been previously described, and disrupt a conserved 16S ribosomal RNA stem structure. The mutation in the SPC23 line is the first reported case of a mutation close to the region of the 16S rRNA involved in the formation of the initiation complex. The new mutants provide markers for selecting plastid transformants.     

Native root-nodule bacteria of traditional soybean-growing areas of northern Thailand

Over 1500 root-nodule bacteria were isolated from a range of uninoculated soybeans, and one cowpea, trap-hosts, sown in 1985 into traditional soybean-growing areas of soybean-growing areas of northern Thailand. Most isolates were slow-growing Bradyrhizobium japonicum. Using a modified bottle-jar technique, 586 of the isolates were tested with a range of soybean hosts and one cowpea host. The results indicated:

Cytology of induced systemic resistance of cucumber to Colletotrichum lagenarium

The infection of cucumber leaves by Colletotrichum lagenarium was studied using cytological methods. Its progress in untreated plants was compared with that in plants in which systemic resistance had been induced by pre-infecting the first true leaf with the same fungus. In induced plants, a reduction of fungal development was observed at the leaf surface, in the epidermis, and in the mesophyll. On the leaf surface, formation of appressoria was slightly reduced. In the epidermis, enhanced formation of papillae beneath appressoria, and possibly increased lignification of entire cells, correlated with reduced development of infection hyphae. Papillae contained callose, identified by staining with aniline-blue fluorochrome and digestion with -1,3-glucanase, as a main structural component. In the mesophyll, reduced fungal development provided evidence for the existence of an additional induced defence reaction. The results imply that preinfection elicited a systemic, multicomponent defence reaction of the host plant against the fungus.Dedicated to the memory of Professor H. Grisebach