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.