Development and characterization of nickel accumulating mutants of Aspergillus nidulans

Stable mutants of Aspergillus nidulans, resistant to 1 mM Ni were developed by step-by-step repeated culturing of the fungus on the medium containing increasing concentrations of nickel chloride. Characterization of mutants could differentiate them into two categories Ni(R) I and Ni(R) II. Each category of mutants exhibited alterations in growth, conidial germination and melanin secretion both in Ni-free and Ni-containing media. Ni(R) II mutants were little slow in growth with sparse mycelia and conidiation but showed high melanin secretion and higher Ni-uptake in comparison to Ni(R) I mutant. Studies involving metabolic and translational inhibitors could prove that Ni-accumulation was biphasic. The initial energy independent surface accumulation was found to be followed by energy dependent intarcellular uptake. Increase in the concentration of the metal in the medium or the time of exposure did not proportionately increase the metal uptake by the mutants. Ni-uptake followed Michaelis-Menton saturation kinetics, which was enhanced under optimum pH of 6.5-7.5 and reduced complexity of the medium due to free availability of ions. Resistance to Ni was found to be constitutive in Ni(R)I mutant, and could be induced in Ni(R)II mutant.