Kinetics of dissociation of reduced nicotinamide adenine dinucleotide phosphate from its complexes with malic enzyme in relation to substrate inhibition and half-of-the-sites reactivity

Malic enzyme of pigeon liver binds NADPH at four equivalent enzyme sites and binds Mn2+ and malate each at two sets of "tight" and "weak" sites with negative cooperativity [Pry, T. A., & Hsu, R. Y. (1980) Biochemistry 19, 951-962]. Stopped-flow studies on the displacement of NADPH from the malate-enzyme complexes E4-NADPH4, E4-Mn2(2+)-NADPH4, E4-Mn2(2+)-NADPH4-dimalate, and E4-Mn2(2+)-NADPH4-tetramalate by large excess NADP+ or its analogue phosphoadenosine(2')diphospho(5')ribose show that NADPH dissociates from the binary complex rapidly with a first-order rate constant of 427 s-1. Dissociation from the ternary E4-Mn2(2+)-NADPH4 complex containing two tightly bound Mn2+ ions can be described by a single first-order process with a rate constant of 135 s-1, or more satisfactorily by two simultaneous first-order processes attributable to the reactions of Mn2+-deficient (k congruent to 427 s-1) and Mn2+-liganded (k = 96 s-1) subunits. The latter equals twice the maximum steady-state turnover rate of 53.2 + 3.0 s-1 assigned to dissociation of the reduced nucleotide from transient E-Mn2+-NADPH, and this 2:1 ratio strongly supports our proposed "half-of-the-sites" model [Hsu, R. Y., & Pry, T. A. (1980) Biochemistry 19, 962-968]. Dissociation from the E4-Mn2(2+)-NADPH4-dimalate complex (k = 100 s-1) follows only the slower process, suggesting that occupancy of malate at two sites tightens enzyme-bound NADPH on the adjacent sites. Binding of malate at two additional weak sites yields E4-Mn2(2+)-NADPH4-tetramalate and a NADPH dissociation rate constant of 2.69 s-1. The 97% decrease in NADPH dissociation parallels the observed 93% maximal inhibition by malate and is the cause of substrate inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electron Microscope Analysis of Partial Denaturation of F Factor Deoxyribonucleic Acid

Partial denaturation pattern of sex factor deoxyribonucleic acid of Escherichia coli was studied by electron microscopy. Clustering of the adenine-plusthymine-rich regions in one part of the molecule was revealed. The positions of these regions were located on the physical map of F by analyzing the partial denaturation pattern of heteroduplexes between F and F-prime factors with various parts of F sequences deleted.     

The microbial ecology evaluation Device mycology spaceflight studies of Apollo 16

Four fungal species were selected as the test systems for the Microbial Ecology Evaluation Device (MEED) of Apollo 16. Fungal cells were exposed to various quantitative and qualitative spaceflight parameters then returned for postflight analyses. Survival rates and phenotype numbers varied according to exposure parameters. Additional studies underway will further identify the spaceflight changes.     

The structure and behavior of the nucleolus organizers in mammalian cells

The regularly occurring secondary constrictions on metaphase chromosomes of mammalian cells prove to be nucleolus organizers as expected. The expression of nucleolus organizers as secondary constrictions, however, varies from cell to cell and from tissue to tissue, including cultivation in vitro. Electron micrographs of the organizer region show that the nucleolus organizer at metaphase is not a constriction. The width of the organizer area is the same as the condensed chromosomal arms; but the filaments, which are the major components of this region, show a diameter of 50–70 Å. The condensed chromosome arms consist of filaments 150–200 Å in diameter. In some mammalian species, structures similar to the nucleolus organizer are located at the end of chromosomes. These may be terminal nucleolus organizers.Supported in part by Research Grants DRG-269 from Damon Runyon Memorial Fund for Cancer Research, E-286 from American Cancer Society, and HD-2590 from National Institutes of Health.     

Crossing over frequency following inbreeding inNeurospora

Crossing over frequency was determined for five marked regions covering most of a chromosome arm. The results of a 12th backcross were compared with previously published results from the noninbred f₁. Exchange frequency increased significantly following inbreeding. Chiasma interference remained positieve, but was somewhat less intense in b₁₂.Supported in part by Public Health Service Research Grants AI 01462 and GM 1882-01, by a National Science Foundation Faculty Fellowship to H. R. C., and by Public Health Service Research Career Award K6-GM-4899 to D. D. P.Technical Paper No. 2054, Oregon Agricultural Experiment Station, Corvallis.