Extracellular calmodulin stimulates light-independent RbcS-GUS expression in suspension-cultured cells of transgenic tobacco

The RbcS genes encode the small subunits of rubisco; the expression of thes e genes is controlled in a light-dependent and independent manner. It has b een reported that intracellular calmodulin (CaM) is involved in light-depen dent RbcS expression. In this report, the role of extracellular CaM in regu lating expression of RbcS in darkness was examined. The time course of expr ession of RbcS-GUS and that of the secretion of CaM in the suspended transg enic tobacco cells in darkness were very similar. Both showed initial incre ase followed by decline with maximum CaM secretion preceding maximum GUS ex pression by 24 h. The concentration of CaM in the culture medium is regulat ed light independently. Purified CaM alone added to the media enhanced RbcS -GUS expression in darkness. The addition of membrane-impermeable CaM inhib itors, such as anti-CaM antiserum or W7-agarose, repressed the expression o f RbcS-GUS in darkness, but this inhibitory effect was completely reversed by adding exogenous purified CaM. These results provide the first evidence that extracellular CaM is involved in the regulation of light-in dependent RbcS gene expression.