Gene Engineering in Yeast for Biodegradation: Immunological Cross-Reactivity Among Cytochrome P-450 System Proteins ofSaccharomyces CerevisiaeandCandida Tropicalis

Identification and characterization of additional members of the cytochrome P450 multigene family CYP52 of Candida tropicalis

Using different DNA probes from the first two previously described alkane-inducible cytochrome P450 genes of the Candida tropicalis CYP52 gene family, we isolated five independent additional members by screening a genomic library under low-stringency conditions. These genes are not allelic variants and, when taken gogether, constitute the largest gene family known in this organism. The seven members of this gene family are located on four different chromosomes and four of them are tandemly arranged on the C. tropicalis genome. The products of the seven genes, alk1 to alk7, were compared to each other and revealed a high degree of divergence: the two most diverged proteins exhibit a sequence identity of only 32%. Six of the seven genes were shown to be induced by a variety of different aliphatic carbon sources but repressed when the organism was grown on glucose. Three of the five additional CYP52 genes could be successfully expressed in Saccharomyces cerevisiae and display different substrate specificities in in vitro assays with model substrates: alk2 and alk3 exhibit a strong preference for hexadecane, while alk4 and alk5 preferentially hydroxylate lauric acid.

Characterization of a second alkane-inducible cytochrome P450-encoding gene, CYP52A2, from Candida tropicalis

A second alkane-inducible cytochrome P450-encoding gene (CYP52A2) from the yeast Candida tropicalis was sequenced and characterized. CYP52A2 is located 1 kb upstream from CYP52A1, the previously characterized P450 gene [Sanglard and Loper, Gene 76 (1989) 121-136] and shows the same orientation. Like CYP52A1, CYP52A2 is induced by growth on alkane. Both promoter regions share repeats of the sequence CATGTGAA that could be of importance for the induction of the two genes. At the amino acid level, alk2 shows an overall identity of 68.2% and an overall similarity of 81.6% to alk1. Regions of high homology between the two proteins are found in the distal and proximal heme binding sites which contain the highly conserved cysteine residue as the fifth ligand to the heme iron. However, marked differences between the two proteins exist at their N-terminal end, which includes the transmembrane domain, and at the putative substrate-binding domain. Upon expression of CYP52A2 in Saccharomyces cerevisiae, alk2 was shown to hydroxylate hexadecane, but had no hydroxylation activity towards lauric acid, whereas alk1 showed both activities. Comparative immunoblots demonstrate that neither alk1 nor alk2 expressed in S. cerevisiae corresponds to the main cytochrome P450 present in C. tropicalis when grown on alkane.

Comparative study on cytochrome P-450 of yeasts using specific antibodies to cytochromes P-450alk and P-450(14DM)

The occurrence of cytochrome P-450(14DM) (lanosterol 14 alpha-demethylase) and cytochrome P-450alk (long-chain alkane terminal hydroxylase) in various yeast strains was determined with immunological procedures. Cytochrome P-450(14DM), which is constitutive or housekeeping enzyme playing an essential role in ergosterol biogenesis, was found in all yeast strains so far tested. Cytochromes P-450(14DM) from different species of yeast were immunologically different, although they may have had a few common antigenic sites. In contrast, cytochrome P-450alk was detected only in the alkane-assimilating yeasts.

Heterogeneity within the alkane-inducible cytochrome P450 gene family of the yeast Candida tropicalis

The reexamination of a genomic lambda gt11 Candida tropicalis expression library for the presence of genes related to the previously reported alkane-inducible cytochrome P450alk gene (P450alk), which is the first member of the P450LII gene family, was undertaken. A positive clone with a DNA fragment having 69% similarity with a portion of P450alk was isolated. As in the case of P450alk, this new putative P450 gene was also induced by tetradecane when C. tropicalis was grown on this carbon source and was therefore named P450alk2, P450alk1 corresponding to the first isolated P450 gene. In addition to P450alk2, the existence of other P450alk-related genes is suggested by the hybridization pattern of P450alk1 and P450alk2 probes with the C. tropicalis genomic DNA. The P450LII gene family in C. tropicalis appears therefore to include several different members. This heterogeneity is presently a unique feature within yeast P450 gene families and resembles the situation existing in P450 gene families of higher eukaryotes.

Disruption of the Saccharomyces cerevisiae gene for NADPH-cytochrome P450 reductase causes increased sensitivity to ketoconazole

Strains of Saccharomyces cerevisiae deleted in the NADPH-cytochrome P450 reductase gene by transplacement are 200-fold more sensitive to ketoconazole, an inhibitor of the cytochrome P450 lanosterol 14 alpha-demethylase. Resistance is restored through complementation by the plasmid-borne wild type gene from either S. cerevisiae or Candida tropicalis. Neither Southern hybridization nor Western immunoblot techniques provided evidence for a second NADPH-cytochrome P450 reductase gene, suggesting that an alternate pathway may provide for the functions of this reductase in S. cerevisiae.

Primary structure of the cytochrome P450 lanosterol 14 alpha-demethylase gene from Candida tropicalis

We report the nucleotide sequence of the gene and flanking DNA for the cytochrome P450 lanosterol 14 alpha-demethylase (14DM) from the yeast Candida tropicalis ATCC750. An open reading frame (ORF) of 528 codons encoding a 60.9-kD protein is identified. This ORF includes a characteristic heme-binding domain, HR2, common to all P450 proteins. This protein and the 14DM from Saccharomyces cerevisiae share 66.5% identical and 23.1% conservatively replaced amino acids in a 516-amino-acid alignment, and thus are orthologous forms of the P450LIA1 gene. Conversely, C. tropicalis 14DM shares relatively little sequence similarity with P450alk, the predominant P450 protein present when this organism is grown on n-alkanes. Sequence information of these three yeast P450s will be useful for structure-function analyses in the future.

Isolation of the Candida tropicalis gene for P450 lanosterol demethylase and its expression in Saccharomyces cerevisiae

We have isolated the gene for cytochrome P450 lanosterol 14 alpha-demethylase (14DM) from the yeast Candida tropicalis. This was accomplished by screening genomic libraries of strain ATCC750 in E. coli using a DNA fragment containing the yeast Saccharomyces cerevisiae 14DM gene. Identity of this gene was confirmed by a) observing a heme binding region common to all P450s after sequencing the 3' portion of the gene, and b) based upon tests of its expression in strains of Saccharomyces cerevisiae.

Isolation of the alkane inducible cytochrome P450 (P450alk) gene from the yeast Candida tropicalis

The gene for the alkane-inducible cytochrome P450, P450alk, has been isolated from the yeast Candida tropicalis by immunoscreening a lambda gt11 library. Isolation of the gene has been identified on the basis of its inducibility and partial DNA sequence. Transcripts of this gene were induced by alkane to levels 500 to 1000 fold over those detected in glucose-grown cells. The nucleotide sequence of the 3' portion of this gene revealed a coding sequence for the heme binding segment characteristic of the P450 gene family.

The distinction of different types of cytochromes P-450 from the yeasts Candida tropicalis and Saccharomyces uvarum

The distinction between two types of cytochromes P-450 originating from microsomes of Candida tropicalis grown on glucose and on alkane was achieved. Criteria of differentiation between these two cytochrome P-450 forms were based on the characteristics of reduced carbon monoxide difference spectra, on substrate specificity, and on binding and inhibition kinetics of the fungistatic compound propiconazole. One cytochrome P-450 form catalyzed the 14 alpha-demethylation of lanosterol and bound propiconazole with an equimolar ratio. This form was present in microsomes from glucose-grown cells and shared similar characteristics with the cytochrome P-450 originating from Saccharomyces uvarum grown on the same carbon source. The other cytochrome P-450 form catalyzed the terminal hydroxylation of aliphatic hydrocarbons and showed a less specific binding ratio with propiconazole (10(3) mol propiconazole for 1 mol cytochrome P-450). This type of cytochrome P-450 was only present in the microsomes of C. tropicalis grown on alkane.

Isolation of a cytochrome P-450 structural gene from Saccharomyces cerevisiae

We have transformed a Saccharomyces cerevisiae host with an S. cerevisiae genomic library contained in the shuttle vector YEp24 and screened the resultant transformants for resistance to ketoconazole (Kc), an inhibitor of the cytochrome P-450 (P-450) enzyme lanosterol 14 alpha-demethylase. Two plasmids were isolated which transformed yeast to both increased resistance to Kc and increased levels of total P-450. Hybrid-selection and immunoprecipitation experiments showed that these plasmids, pVK1 and pVK2, contained the structural gene for an S. cerevisiae P-450. This conclusion was confirmed by the nucleotide sequence of a portion of pVK2, which revealed an open reading frame encoding a characteristic P-450 heme-binding region.