Regulation of expression by divalent cations of a light-inducible gene in Arthrobacter photogonimos

Abstract

Expression of the light-inducible (lipA) gene in Arthrobacter photogonimos is repressed by Ca2+ at a concentration greater than 0.1 μM. Expression of lipA was induced by relatively high concentrations of Zn2+ Ni2+ or Co2+ in cell suspensions, an effect that was blocked by an increase in the concentration of Ca2+ in the medium. Zn2+ and other metals apparently overcame repression by Ca2+ by competing for a cellular binding site. Expression of lipA was also induced when the amount of free Ca2+ was lowered with ethylene-bis (oxyethylenenitrilo)tetraacetic acid (EGTA). Our results show that the lipA gene does not require Zn2+ or other divalent cation for expression and that it is regulated negatively by Ca2+. Accumulation of the mature product of this gene (light-inducible protein, LIP) was minimal in the presence of EGTA. Accumulation increased 10-to 20-fold when divalent cations such as Ca2+, Mn2+, Cu2+ or Zn2+ were added to cell suspensions treated with chelator. These divalent cations, which allowed the protein to achieve a protease-resistant form on the cell surface, could be substituted by protease inhibitors such as antipain, leupeptin or 1,10-phenanthroline. Our data can be explained by a biparous mechanism in which divalent cations regulate both expression of the lipA gene and accumulation of the gene product.