No abstract

Type-II NAD(P)H dehydrogenases form a multigene family that comprise six members in the green microalga Chlamydomonas. To date, only one enzyme (Nda2) located in the chloroplast has been characterized in this alga and demonstrated to participate in the reduction of the plastoquinone pool. We present here the functional characterization of Nda1. The enzyme is located on the inner face of the inner mitochondrial membrane. Its downregulation leads to a slight decrease of NADH:ferricyanide activity and of dark whole cell respiration. To determine whether the reduction of Nda1 combined with the lack of complex I would affect mitochondrial processes, double mutants affected in both Nda1 and complex I were isolated. Respiration and growth rates in heterotrophic conditions were significantly altered in the double mutants investigated, suggesting that Nda1 plays a role in the oxidation of matrix NADH in the absence of complex I.

In photosynthetic eukaryotes, the redox state of the plastoquinone (PQ) pool is an important sensor for mechanisms that regulate the photosynthetic electron transport. In higher plants, a multimeric nicotinamide adenine dinucleotide (phosphate) (NAD(P))H dehydrogenase (NDH) complex and a plastid terminal oxidase (PTOX) are involved in PQ redox homeostasis in the dark. We recently demonstrated that in the microalgae Chlamydomonas reinhardtii, which lacks the multimeric NDH complex of higher plants, non-photochemical PQ reduction is mediated by a monomeric type-II NDH (Nda2). In this study, we further explore the nature and the importance of non-photochemical PQ reduction and oxidation in relation to redox homeostasis in this alga by recording the 'dark' chlorophyll fluorescence transients of pre-illuminated algal samples. From the observation that this fluorescence transient is modified by addition of propyl gallate, a known inhibitor of PTOX, and in a Nda2-deficient strain we conclude that it reflects post-illumination changes in the redox state of PQ resulting from simultaneous PTOX and Nda2 activity. We show that the post-illumination fluorescence transient can be used to monitor changes in the relative rates of the non-photochemical PQ reduction and reoxidation in response to different physiological situations. We study this fluorescence transient in algae acclimated to high light and in a mutant deficient in mitochondrial respiration. Some of our observations indicate that the chlororespiratory pathway participates in redox homeostasis in C. reinhardtii.

No abstract

Pichia pastoris is a very popular yeast for recombinant protein production, mainly due to the strong, methanol-inducible P AOX1 promoter. Methanol induction however poses several drawbacks. One approach to improve processes is to use MutS strains with reduced methanol catabolic ability. Various reports claim that MutS allows higher recombinant protein production levels than Mut+ but scarcely elaborate on reasons for differences. In this study, enhanced green fluorescent protein was used as a P AOX1 -driven reporter for the investigation of expression differences between Mut+ and MutS strains. Mut+ exhibited higher responses to methanol, with faster growth (0.07 vs. 0.01 hr −1 ) and higher consumption of methanol (2.25 vs.1.81 mmol/g DCW .hr) and oxygen (2.2 vs. 0.66 mmol/g DCW .hr) than MutS. Mut+ yielded more biomass than MutS (2.3 vs. 1.3 g DCW /L), and carbon dioxide analysis of bioreactor off-gas suggested that considerable amounts of methanol were consumed by Mut+ via the dissimilatory pathway. In contrast, it was demonstrated that the MutS population switched to an induced state more rapidly than Mut+. In addition, MutS exhibited 3.4-fold higher fluorescence levels per cell (77,509 vs. 23,783 SFU) indicative of higher recombinant protein production. The findings were verified by similar results obtained during the expression of a lipase. Based on the differences in response to methanol versus recombinant protein production, it was proposed that higher energy availability occurs in MutS for recombinant protein synthesis, contrary to Mut+ that uses the energy to maintain high levels of methanol catabolic pathways and biomass production.