Date of Award

Spring 1989

Project Type

Dissertation

Program or Major

Microbiology

Degree Name

Doctor of Philosophy

Abstract

Magnetosomes were purified from broken cells of Aquaspirillum magnetotacticum by a magnetic separation technique. Electron microscopic and chemical analyses revealed they consisted of crystalline magnetite (Fe$\sb3$O$\sb4$) cores enclosed by a lipid bilayer membrane containing numerous proteins. Two membrane proteins were absent from the nonmagnetic (membrane or soluble) cell fractions and were absent from cells of non-magnetic mutant strains. Knowledge of their partial sequence is expected to lead to construction of hybridization probes to identify magnetosome-specific DNA sequences.

Cells of this organism in laboratory culture typically achieved low cell yields and were variably magnetic or non-magnetic. Therefore better understanding of physiological conditions appropriate for growth and Fe$\sb3$O$\sb4$ production was a prerequisite for studies of molecular biology and membrane biogenesis. The cell and Fe$\sb3$O$\sb4$ yield in batch-cultured denitrifying cells was previously shown to depend upon culture dissolved oxygen tension (D.O.T.). We developed continuous culture methods for this obligate microaerophile and obtained, with D = 0.075$\sp{-}$h (Td = 9.2 h), control of pH (6.8), and D.O.T. control (1-10% of saturation), 10-fold higher cell yields with more predictable and reproducible cell growth and Fe$\sb3$O$\sb4$ production than were previously possible. Our interest has been to use continuous culture to further clarify the effect of D.O.T. in magnetosome biogenesis.

Denitrifying magnetic cells (4 mM No$\sb3\sp{-}$ as the limiting nutrient) and non-denitrifying cells (0.1 mM NH$\sb4\sp{+}$ as the limiting nutrient) became non-magnetic as the D.O.T. was increased from 1 to 5%. Although 5% O$\sb2$ was toxic for denitrifying cells and cultures washed out, non-denitrifying cultures again became magnetic when the D.O.T. was decreased to 1%. This reversible transition between magnetic and non-magnetic state, regulated by D.O.T., provided a reproducible system for examining magnetosome biogenesis.

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