Transcriptional Regulation of Metabolic Genes by the Basic Leucine Zipper Transcription Factor Hac1ip and Nutrient Stimuli

Saccharomyces cerevisiae cells respond to nutrients in their environment by altering their metabolic and transcriptional state in order to optimise the use of available nutrients and decide which of the several developmental pathways to pursue. In the yeast S. cerevisiae, meiosis and pseudohyphal growth are two major differentiation outcomes in response to nitrogen starvation. A central component of unfolded protein response pathway, the bZIP transcription factor Hac1ip, negatively regulates meiosis and pseudohyphal growth. The present study investigates this negative regulatory mechanism at early meiotic genes by Hac1ip in nitrogen-rich conditions. Regulation of transcription by Ume6p transcriptional regulator, Rpd3p-Sin3p histone deacetylase complex and Isw2p-Itc1p chromatin remodelling complex at URS1 was also investigated here. We also tested for induction of pseudohyphal growth in diploids from SK1 genetic background in response to nitrogen starvation conditions known to induce meiosis. I constructed destabilised β-galactosidase reporters expressed from URS1- CYC1-Ub-X-lacZ reporters to analyze transcriptional activity at URS1 site of early meiotic genes in nutrient rich conditions. The data presented here successfully demonstrates Hac1ip-mediated repression at URS1 sites in nitrogen-rich conditions. URS1-CYC1-Ub-X-lacZ reporters were expressed in mitotic repression machinery mutants (ume6Δ, rpd3Δ, sin3Δ, isw2Δ and itc1Δ) under nitrogen rich conditions. The data presented here from these experiments not only corroborates their known role in repression at URS1 but also suggested regulation at additional sites in the minimal CYC1 promoter. Deletion of Sin3p suggested independent repression function separable from Rpd3p. Isw2p also acts independently of Itc1p at sites other than URS1. We also show that pseudohyphal growth was stimulated by non-fermentable carbon sources in sporulation efficient SK1 genetic background. The data also indicates that stimulation of pseudohyphal growth by non-fermentable carbon sources does not require respiration function or functional mitochondrial RTG pathway

The human mitochondrial enzyme BCO2 exhibits catalytic activity toward carotenoids and apocarotenoids.

The enzyme β-carotene oxygenase 2 (BCO2) converts carotenoids into more polar metabolites. Studies in mammals, fish, and birds revealed that BCO2 controls carotenoid homeostasis and is involved in the pathway for vitamin A production. However, it is controversial whether BCO2 function is conserved in humans, because of a 4-amino acid long insertion caused by a splice acceptor site polymorphism. We here show that human BCO2 splice variants, BCO2a and BCO2b, are expressed as pre-proteins with mitochondrial targeting sequence (MTS). The MTS of BCO2a directed a green fluorescent reporter protein to the mitochondria when expressed in ARPE-19 cells. Removal of the MTS increased solubility of BCO2a when expressed in Escherichia coli and rendered the recombinant protein enzymatically active. The expression of the enzymatically active recombinant human BCO2a was further improved by codon optimization and its fusion with maltose-binding protein. Introduction of the 4-amino acid insertion into mouse Bco2 did not impede the chimeric enzymes catalytic proficiency. We further showed that the chimeric BCO2 displayed broad substrate specificity and converted carotenoids into two ionones and a central C14-apocarotendial by oxidative cleavage reactions at C9,C10 and C9,C10. Thus, our study demonstrates that human BCO2 is a catalytically competent enzyme. Consequently, information on BCO2 becomes broadly applicable in human biology with important implications for the physiology of the eyes and other tissues