The regulation of poly- -hydroxybutyrate metabolism in Azospirillum brasilense during balanced growth and starvation

Abstract

Activities of the enzymes which are involved in the poly-P-hydroxybutyrate (PHB) cycle, in both synthesis and degradation reactions, were assayed in crude extracts of Azospirilum brasilense cells containing different amounts of PHB. The enzymes of the PHB cycle, of both the synthesis and the degradation process, were more active in PHB-rich cells than in PHB-poor cells. During 96 h of starvation of cells suspended in phosphate buffer, enzymes of the PHB cycle were more active in PHB-rich cells. There was a peak of activity of hydroxybutyrate dehydrogenase (BOHB-DH), P-ketothiolase and thiophorase after 24 h of starvation, due to polymer degradation. During the following hours of starvation there was a decrease in the activity of these enzymes. After 24 h of starvation the activity of acetoacetyl-CoA reductase dropped to a minimum level, because the cells could not synthesize PHB under these conditions. The specific activities of BOHB-DH, P-ketothiolase and thiophorase were higher in A. brasiZense cells which were grown under low oxygen tension and consequently accumulated high levels of PHB, than in cells grown under high oxygen tension, with a low PHB content. Similarities to the pathway of PHB biosynthesis and degradation and its control in Azofobacfer beijevnckii are described.

Energy Taxis Is the Dominant Behavior in Azospirillum brasilense

“…During prolonged growth in a rich medium, A. brasilense cells accumulate PHB as an energy and/or carbon storage material (35)(36)(37). PHB oxidation involves a specific NADH-dependent dehydrogenase (35), which competes with the TCA cycle intermediates for the electron transport system (36).…”

“…PHB oxidation involves a specific NADH-dependent dehydrogenase (35), which competes with the TCA cycle intermediates for the electron transport system (36). The presence of intracellular PHB in the cells (growth above OD 600 ϭ 0.5) prevented chemotaxis to any chemoattractant (Table 3), while the cells remained fully motile and able to respond to repellents.…”

“…b Cells were grown to the late exponential growth stage (OD 600 ϭ 0.8) under high aeration in order to achieve high levels of the intracellular PHB (35,36).…”

“…Similarly, inhibitors of the metabolism of a substrate specifically inhibit chemotaxis toward this substrate. The presence of an intracellular metabolizable substrate, PHB, which competes with metabolizable attractants for the electron transport system (35,36), prevents chemotaxis to any extracellular attractant. (ii) There is a direct correlation between the efficiency of a chemical as a growth substrate and as a chemoeffector: the most efficient growth substrates are the stronger attractants for A. brasilense.…”

Motility, chemokinesis, and methylation-independent chemotaxis in Azospirillum brasilense

“…A. brasilense has been shown to start utilizing the endogenous storage substrate poly-p-hydroxybutyrate (PHB) after a few hours of starvation (44). It is possible that the ASH+ fell below the saturation level for flagellar rotation immediately after the bacteria were resuspended in buffer because of the lack of respiratory substrates, causing a consequential decrease in swimming speed.…”