Characterization of the gene for the chromosomal dihydrofolate reductase (DHFR) of Staphylococcus epidermidis ATCC 14990: the origin of the trimethoprim-resistant S1 DHFR from Staphylococcus aureus?

The gene for the chromosomally encoded dihydrofolate reductase (DHFR) of Staphylococcus epidermidis ATCC 14990 has been cloned and characterized. The structural gene encodes a polypeptide of 161 amino acid residues with a calculated molecular weight of 18,417. This trimethoprim-sensitive (Tmps) DHFR, SeDHFR, differs in only three amino acids (Val-31-->Ile, Gly-43-->Ala, and Phe-98-->Tyr) from the trimethoprim-resistant (Tmpr) S1 DHFR encoded by transposon Tn4003. Since in addition the S. epidermidis gene also forms part of an operon with thyE and open reading frame 140 as in Tn4003, the chromosomally located gene encoding the Tmps SeDHFR is likely to be the molecular origin of the plasmid-located gene encoding the Tmpr S1 DHFR. Site-directed mutagenesis and kinetic analysis of the purified enzymes suggest that a single Phe-->Tyr change at position 98 is the major determinant of trimethoprim resistance.     

Nucleotide variation at the hypervariable esterase 6 isozyme locus of Drosophila simulans

Esterase 6 (Est-6/EST6) is polymorphic in both Drosophila melanogaster and D. simulans for two common allozyme forms, as well as for several other less common variants. Parallel latitudinal clines in the frequencies of the common EST6-F and EST6-S allozymes in these species have previously been interpreted in terms of a shared amino acid polymorphism that distinguishes the two variants and is subject to selection. Here we compare the sequences of four D. simulans Est-6 isolates and show that overall estimates of nucleotide heterozygosity in both coding and 5' flanking regions are more than threefold higher than those obtained previously for this gene in D. melanogaster. Nevertheless, the ratio of replacement to exon silent-site polymorphism in D. simulans is less than the ratio of replacement to silent divergence between D. simulans and D. melanogaster, which could be the result of increased efficiency of selection against replacement polymorphisms in D. simulans or to divergent selection between the two species. We also find that the amino acid polymorphisms separating EST6- F and EST6-S in D. simulans are not the same as those that separate these allozymes in D. melanogaster, implying that the shared clines do not reflect shared molecular targets for selection. All comparisons within and between the two species reveal a remarkable paucity of variation in a stretch of nearly 400 bp immediately 5' of the gene, indicative of strong selective constraint to retain essential aspects of Est-6 promoter function.      

The effect of manganese oxides and manganese ion on growth and siderophore production by Azotobacter vinelandii

The addition of manganese oxides to iron-limited medium promoted the formation of the pyoverdin siderophore azotobactin by Azotobacter vinelandii. When active-MnO₂ was used, there was greatly decreased iron uptake into the cells, hyperproduction of azotobactin and the abiotic, chemical destruction or adsorbtion of the catechol siderophores azotochelin and aminochelin by this strong oxidizing agent. Although the iron content of the cells was the same as iron-limited cells, the growth of cells in medium with active-MnO₂ was increased 1.5- to 2.5-fold over iron-limited controls. This growth promotion was not caused by iron contaminating the oxide or by manganese solubilized from the oxide. Soluble 0.05–4 mm Mn²⁺ inhibited the growth of iron-limited cells and had a minimal effect on iron uptake, but caused hyperproduction of azotobactin. This later effect was caused by the inhibition of soluble ferric reductase, in a manner identical to that previously observed for Zn²⁺. These results suggest that active-MnO₂ may interfere with a surface-localized iron uptake site, possibly another ferric reductase. The reason for the growth promotion by active-MnO₂ remains unknown, but is most likely related to decreased oxygen toxicity.     

Elastin cross-linking in vitro. Studies on factors influencing the formation of desmosines by lysyl oxidase action on tropoelastin.

The formation of isodesmosine and desmosine in vitro by the action of lysyl oxidase on tropoelastin was studied. The synthesis of desmosines occurred in the absence of additional substances. The formation of desmosines was not affected by removal of molecular O2 from the reaction medium nor was it affected by the lack of proline hydroxylation in tropoelastin. However, there was virtually no desmosine formation at 15 degrees C, a temperature not conducive to coacervation, indicating that coacervation is an important prerequisite for cross-linking.     

Reciprocal translocation and the Philadelphia chromosome

Summary We examined metaphases from three patients with chronic myeloid leukaemia and a typical Philadelphia chromosome with one chromosome 9 as the recipient to determine whether the 9q+ 22q- translocation is reciprocal. Good quality G-banded photographs of the chromosomes concerned were subjected to light absorption density analysis. This provided enlarged tracings corresponding to the relevant chromosome regions and so facilitated accurate measurement. This technique has unambiguously shown that the typical Philadelphia chromosome results from a reciprocal translocation and that probably no material is gained or lost in the exchange. Furthermore, in a total of six patients for whom sequential G and C banding was performed, the chromosome 9 with the largest block of centromeric heterochromatin received the translocated material. We offer tentative explanations for this curious observation.     

Free amino acids in brain, liver, and skeletal muscle tissue of voles infected with Trypanosoma brucei gambiense.

The concentrations of several acidic and neutral amino acids of brain, liver, and skeletal muscle were determined in field voles, Microtus montanus, and compared to values obtained from voles harboring a chronic infection of Trypanosoma brucei gambiense. All of the amino acids examined were found at comparable levels in brain tissue from both groups of animals with the exception of tyrosine, which was reduced by approximately 45% in the infected voles. Similarly, the only difference noted in liver tissue was 32% decrease of free tyrosine in the infected animals. With respect to muscle tissue, in addition to a 45% reduction of free tyrosine in the infected voles, decreases of a smaller magnitude were also noted for threonine, glutamate, and valine. The relatively specific alteration of free tyrosine concentrations in the investigated tissues of trypanosome-infected animals suggests an alteration in host metabolism of this amino acid and/or parasite utilization.