Site-directed mutagenesis of the katG gene of Mycobacterium tuberculosis: effects on catalase-peroxidase activities and isoniazid resistance.

Recent studies examining the molecular mechanisms of isoniazid (INH) resistance in Mycobacterium tuberculosis have demonstrated that a significant percentage of drug-resistant strains are mutated in the katG gene which encodes a catalase-peroxidase, and the majority of these alterations are missense mutations which result in the substitution of a single amino acid. In previous reports, residues which may be critical for enzymatic activity and the drug-resistant phenotype have been identified by evaluating INH-resistant clinical isolates and in vitro mutants. In this study, site-directed mutagenesis techniques were utilized to alter the wild-type katG gene from M. tuberculosis at 13 of these codons. The effects of these mutations were determined using complementation assays in katG-defective, INH-resistant strains of Mycobacterium smegmatis and Mycobacterium bovis BCG. This mutational analysis revealed that point mutations in the katG gene at nine of the 13 codons can cause drug resistance, and that enzymatic activity and resistance to INH are inversely related. In addition, mutations in the mycobacterial catalase-peroxidase which reduce catalase activity also decrease peroxidase activity.

Mental health under national health care reform: the empirical foundations.

This article reviews research pertinent to mental health services under several U.S. health care reform proposals. Issues examined include the redistributional impact of the inclusion of outpatient mental health benefits, optimal benefit packages, and findings that mental health services lower medical utilization costs. It is argued that extending a minimalist model of time-limited benefits, similar to that implemented in most managed care programs, to a national health care insurance plan would perpetuate the current two-class mental health care system.

Oligodendrocyte precursor quantitation and localization in perinatal brain using a retrospective bioassay.

Several late stages of the oligodendrocyte (OL) developmental lineage can be identified immunologically in the newborn rat brain. However, OL lineage-specific markers are not available for the detection of the less mature, yet determined, OL precursors. We have developed a retrospective bioassay, combining limiting dilution analysis with a novel culture system, that quantitatively assesses the developmental potential in vivo of phenotypically undefined OL precursors in order to (1) demonstrate their existence, (2) estimate their total number in the premyelinated rat brain, and (3) demonstrate their presence in regions distal to germinal zones at times previously predicted to be devoid of such cells. Between embryonic day (E) 21 and postnatal day (P) 0, cells determined to become oligodendrocytes increase in frequency approximately 5-fold in the whole brain (from one precursor for every 365 cells to 1 in 74), and approximately 2.5-fold in the telencephalon (from 1 in 298 to 1 in 115). From these data it is calculated that a pool of approximately 10(6) phenotypically undefined cells are present in the newborn brain that are able to differentiate into OL in vitro. Further, by applying this assay to tissue samples of subdomains of the developing cerebellum, we have demonstrated that such cells are present in large numbers as early as E20 in regions sparsely populated with cells expressing the blastic neural cell marker ganglioside GD3, suggesting that they migrated to this position as a pre-GD3-expressing cell. These results significantly change the predicted ontogeny of the oligodendrocyte lineage and should fuel the ongoing search for these early OL precursors.