Elevated mRNA levels of the ribosomal protein L19 and a calmodulin-like protein in assimilate-accumulating transgenic tobacco plants

Communication between sink and source organs is crucial for normal plant development. The synthesis of assimilates during photosynthesis must be adapted to the demand in sink tissues. Surplus of carbon dioxide assimilation in source leaves leads to the accumulation of soluble sugars in mesophyll cells and thereby to the inhibition of photosynthesis. The underlying mechanism of the so called "sink limitation" of photosynthesis is completely unknown. To investigate the molecular changes associated with assimilate accumulation, we made use of transgenic tobacco (Nicotiana tabacum) plants constitutively accumulating soluble sugars in their source leaves. Differential hybridization was used to isolate several cDNA clones. The protein products of two of these clones, which are induced under conditions of sugar accumulation, were identified. One of these cDNA contains an open reading frame of 211 amino acids, which shares striking homology with the ribosomal protein L19 from Dictyostelium (55% identity) and human (63% identity). The other cDNA clone codes for a polypeptide of 210 amino acids, which shows 39% identity with and 39% similarity to a calmodulin-like protein from Arabidopsis. Calmodulins belong to the superfamily of Ca super(2+)-binding EF-hand proteins. Within the protein there are two domains encoding typical and two domains encoding similar EF-hand sequences.