Increased diuron resistance in the joint expression of mutations located at the DIU2, DIU3 and DIU4 loci of Saccharomyces cerevisiae

Construction of insertion mutants of Synechocystis sp. PCC 6803: evidence for an essential function of subunit IV of the cytochrome b6/f complex

The gene encoding subunit IV of the cytochrome b6/f complex (petD) has been isolated from a genomic library of the unicellular cyanobacterium Synechocystis sp. PCC 6803. The coding region consists of 480 nucleotides and can code for a polypeptide with a molecular weight of 17.5 kDa. The deduced amino acid sequence shows high identity with the corresponding sequences of both the photoautotrophic prokaryote Nostoc sp. PCC 7906 as well as of lower and higher photoautotrophic eukaryotes (e.g. Chlorella prototecoides, Nicotiana tabacum). Transformation of Synechocystis sp. PCC 6803 with a plasmid containing the cloned petD gene in which the coding sequence is interrupted by the aminoglycoside 3'-phosphotransferase gene (aph) from Tn903 resulted in the formation of km resistant transformants. The molecular analysis of independent transformants revealed that all clones were merodiploid containing both uninterrupted wild-type as well as interrupted mutant petD copies. Approaches to segregate these two genomes were unsuccessful implying an essential function of the petD gene product in Synechocystis sp. PCC 6803.

Nuclearly inherited diuron-resistant mutations conferring a deficiency in the NADH--or succinate--ubiquinone oxidoreductase activity in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, diuron, antimycin and myxothiazol block the respiratory pathway at the bc1 complex level. Nuclearly inherited mutations located at the DIU3 and DIU4 loci confer in vitro resistance to diuron and cross-resistance to antimycin and myxothiazol at the NADH oxidase level. The mutant strains do not exhibit diuron resistance at the quinol-cytochrome-c oxidoreductase level. Thus, the apparent resistance does not seem to be the result of a modification of the inhibitory sites. Instead, the quinone reduction rate was found to be altered in the mutant. The diu3 mutations lead to a deficiency of the NADH--ubiquinone oxidoreductase activity, and the diu4 mutations to a deficiency of the succinate--ubiquinone oxidoreductase activity. On the basis of the model of Kröger and Klingenberg, a decrease of quinone reduction could explain the resistance to the bc1 complex inhibitors. Thus, the apparent resistance to the bc1 complex inhibitors was found to be due to a modification of the electron transfer kinetics.