Autotrophic and Heterotrophic Metabolism of Hydrogenomonas: Regulation of Autotrophic Growth by Organic Substrates

Abstract

The effects of a number of organic substrates on the autotrophic metabolism of Hydrogenomonas eutropha were examined. Dual substrate (mixotrophic) cultivation in the presence of hydrogen plus either fructose or alanine allowed autotrophic growth to begin immediately after the exhaustion of the organic substrate. On the other hand, the presence of acetate, pyruvate, or glutamate caused a lengthy lag to occur before autotrophic growth commenced. With acetate or pyruvate this lag (plateau) in the dicyclic growth curve was due to the repression of ribulose diphosphate carboxylase (RDPC) synthesis during mixotrophic growth. During heterotrophic growth with glutamate, RDPC was partially repressed; however, during mixotrophic growth, RDPC activity was high. Thus the delay of autotrophic growth was not due to a repression of RDPC by glutamate. The data suggest that glutamate interferes with autotrophic metabolism by repressing the incorporation of inorganic nitrogen. The repression of these vital autotrophic functions by acetate, pyruvate, and glutamate occurred both in the presence and absence of hydrogen, i.e., during both heterotrophic and mixotrophic cultivation. The derepression of the affected systems during the plateau phase of the dicycic growth curves was demonstrated. Carbon dioxide assimilation by whole cells agreed well with the RDPC activity of extracts from cells grown under similar conditions. 339 on July 16, 2020 by guest

Hydrogenase and ribulose diphosphate carboxylase during autotrophic, heterotrophic, and mixotrophic growth of scotochromogenic mycobacteria

“…Since 7H9 broth allows much faster heterotrophic growth than any single organic substrate tested, this supports those observations (6) as well as growth results obtained with A. eutrophus (5). However, cultures of all three autotrophic mycobacteria grown mixotrophically in 7H9 broth contained moderate levels of both autotrophic enzymes, in contrast to the severely repressed levels of RuDP carboxylase in A. eutrophus during rapid heterotrophic growth (27). Reducing the oxygen tension did not stimulate either enzyme system in mycobacteria, as it does in some other hydrogen autotrophs.…”

“…Mixotrophic incubation with acetate and pyruvate elicited very different responses in M. gordonae as compared with Alcaligenes eutrophus (Hydrogenomonas eutropha). With A. eutrophus the presence of acetate or pyruvate completely repressed RuDP carboxylase synthesis (27). In M. gordonae cells grown mixotrophically, these substrates yielded carboxylase levels comparable to those in cells grown in the presence of other organic substrates, and both autotrophic and heterotrophic systems functioned simultaneously, producing greater growth rates and yields than with either heterotrophic or autotrophic cultures.…”

“…The RuDP carboxylase systems in the facultative autotrophs all seem to be inducible or repressible and appear to be a major mechanism regulating autotrophic metabolism. Physical factors such as oxygen tension (13,23,29) as well as the type of organic substrate available (13,16,21,27) can affect the levels of hydrogenase and RuDP carboxylase in these organisms. The repressive effect of acetate and pyruvate for A. eutrophus was shown to be at the level of enzyme synthesis and not a direct inhibition of enzyme activity (27).…”

“…Physical factors such as oxygen tension (13,23,29) as well as the type of organic substrate available (13,16,21,27) can affect the levels of hydrogenase and RuDP carboxylase in these organisms. The repressive effect of acetate and pyruvate for A. eutrophus was shown to be at the level of enzyme synthesis and not a direct inhibition of enzyme activity (27). DeCicco and Umbreit (6) suggested that those organic substrates that are utilized relatively slowly allow simultaneous autotrophic and heterotrophic metabolism, whereas more readily utilizable organic substrates repress autotrophic metabolism.…”

“…RuDP carboxylase assay. The method employed for the RuDP carboxylase assay has been described previously (27). A unit of enzyme activity is defined as the amount catalyzing the formation of 1 nmol of 3-phosphoglyceric acid per min.…”