Response of Rhizobium leguminosarum to nickel stress

Rhizobium leguminosarum strain P-5 biovar viciae was sensitive to Ni2+ (MIC , 75 muM) and showed concentration-dependent Ni2+ uptake in a wide concentr ation range (50-500 muM). Ni2+ uptake up to a certain threshold limit also increased thiol content (66 nmol mg(-1) protein), proline content (10.85 nm ol mg(-1) protein) and urease specific activity (500 nmol min(-1) mg(-1) pr otein) maximum corresponding to 100 muM Ni2+ as the external concentration or 151 nmol Ni2+ mg(-1) protein as the intracellular buildup. Proline synth esis was stimulated most even at much lower Ni2+ concentration (25 muM). Hi gher intracellular Ni2+ load neither favoured thiol nor proline biosynthesi s nor urease activity. Ni2+ requirement of urease was ascertained by using EDTA-grown cells and the addition of bicarbonate (NaHCO3, 100 mM) to the cr ude extract. The induction of thiol or proline by Ni2+, therefore, reflects the possible strategies adopted by bacterial cells to overcome the environ mental stress.