ZINC RESISTANCE IN NEUROSPORA-CRASSA

Three non-identical Zn2+-resistant strains of Neurospora crassa have b een isolated, ZNR-1 and ZNR-2 strains were obtained after repeated sub culturing of wild type N, crassa on Zn2+-containing agar media (8 mM a nd 16 mM), while ZNR-3 was isolated after mutagenesis with diethyl sul fate, followed by selection on Zn2+ agar plates (16 mM). All three ZNR strains showed two- to threefold resistance to Zn2+ in liquid media w hen compared with the wild type, However, growth measured by hyphal el ongation clearly distinguished between the resistant strains (ZNR-3 > ZNR-2 > ZNR-1 >>> wild), The ZNR-2 and ZNR-3 strains were also cross-r esistant to Co2+, while ZNR-2 alone was cross-resistant to Cu2+, Both Mg2+ and Fe3+ reversed the growth inhibition caused by Zn2+; Mg2+ by s uppression of Zn2+ uptake and Fe3+ without affecting the same, Assay o f catalase, iron-binding siderophores and glutathione in Zn2+ toxicity revealed significant increases in catalase and glutathione levels in the ZNR-2 strain when compared with the wild type, Kinetics of Zn2+ up take by preformed mycelia showed a rapid initial phase of uptake follo wed by a slower phase, The rates of Zn2+ uptake measured after leachin g surface-bound metal with EI)TA revealed that ZNR strains have signif icantly reduced Zn2+ uptake rates when compared with the wild type. Th e overall data suggest a partial transport block for Zn2+ uptake as th e major mechanism for resistance in ZNR strains, Genetic analysis of Z NR strains showed that in the ZNR-3 strain the znr locus maps close to the mating type locus (mt) of N. crassa LG I, while that of ZNR-1 and ZNR-2 is linked to LG IV associated,vith chromosomal aberration.