Academic journal article Genetics

Genetic Analysis of the Role of Peroxisomes in the Utilization of Acetate and Fatty Acids in Aspergillus Nidulans

Academic journal article Genetics

Genetic Analysis of the Role of Peroxisomes in the Utilization of Acetate and Fatty Acids in Aspergillus Nidulans

Article excerpt

ABSTRACT

Peroxisomes are organelles containing a diverse array of enzymes. In fungi they are important for carbon source utilization, pathogenesis, development, and secondary metabolism. We have studied Aspergillus nidulans peroxin (pex) mutants isolated by virtue of their inability to grow on butyrate or by the inactivation of specific pex genes. While all pex mutants are able to form colonies, those unable to import PTS1 proteins are partially defective in asexual and sexual development. The pex mutants are able to grow on acetate but are affected in growth on fatty acids, indicating a requirement for the peroxisomal localization of β-oxidation enzymes. However, mislocalization of malate synthase does not prevent growth on either fatty acids or acetate, showing that the glyoxylate cycle does not require peroxisomal localization. Proliferation of peroxisomes is dependent on fatty acids, but not on acetate, and on PexK (Pex11), expression of which is activated by the FarA transcription factor. Proliferation was greatly reduced in a farAΔ strain. A mutation affecting a mitochodrial ketoacyl-CoA thiolase and disruption of a mitochondrial hydroxy-acyl-CoA dehydrogenase gene prevented growth on short-chain but not long-chain fatty acids. Together with previous results, this is consistent with growth on even-numbered short-chain fatty acids requiring a mitochondrial as well as a peroxisomal β-oxidation pathway. The mitochondrial pathway is not required for growth on valerate or for long-chain fatty acid utilization.

EUKARYOTES contain single membrane organelles called microbodies containing specialized enzymes involved in a wide range of metabolic activities. Commonly these enzymes include oxidases generating hydrogen peroxide. When this occurs, the microbodies contain catalase and other antioxidative activities to remove reactive oxygen species and they are termed peroxisomes. In some organisms and plant tissues, microbodies lack catalase but contain enzymes of the glyoxylate bypass and these microbodies are often called glyoxysomes (for reviews see Titorenko and Rachubinski 2001; Platta and Erdmann 2007). In the filamentous fungus Neurospora crassa, microbodies contain glyoxylate cycle enzymes but lack catalase as well as peroxisomal oxidases but also contain enzymes for fatty acid β-oxidation with acyl-CoA dehydrogenase substituting for acyl-CoA oxidase(Kionka and Kunau 1985; Thieringer and Kunau 1991; Gainey et al. 1992; Schliebs et al. 2006). It is unlikely that the two classes of microbodies differ fundamentally and this is supported by the finding in silico of conserved orthologs of the full range of proteins that constitute peroxisomes (peroxins) in N. crassa(Kiel et al. 2006). This is also likely to be the case for plants (e.g., Nishimura et al. 1986).

Peroxins are proteins required for peroxisome division, for biogenesis from the endoplasmic reticulum, and for the import of proteins into the peroxisomal matrix (reviewed by Platta and Erdmann 2007). Mutations in pex genes can result in the absence of peroxisomes, abnormal peroxisomal structures, mistargeting of matrix proteins, or an inability to respond to stimuli that cause increased numbers of peroxisomes. Two major classes of peroxisomal targeting signals occur in matrix proteins. PTS1 sequences comprise three Cterminal amino acids (aa) usually of the form S/A R/ K L/M although the context of the C-terminal sequence can greatly affect targeting (Brocard andHartig 2006) and some peroxisomal proteins have cryptic PTS1 sequences (e.g., Klein et al. 2002). Other matrix proteins have PTS2 sequences close to the N terminus with the consensus R/K L/V/I X5 H/Q L/A/F/I (Petriv et al. 2004). A large number of peroxins are involved in the import of all matrix proteins while others are specific to each PTS class; e.g., Pex5 and Pex7 are the specific receptors for PTS1 and PTS2 proteins, respectively (Lazarow 2006; Stanley and Wilmanns 2006).

Many microorganisms are able to use two carbon compounds and fatty acids as sole carbon sources. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.