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 (von Schaewen et al., 1990; Sonnewald et al., 1991; Sonnewald, 1992). Differential hybridization was used to isolate severa1 cDNA clones (Table I). 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; Singleton et al., 1989; Singleton et al., 1989; Kumabe et al., 1992). 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39% identity with and 39% similarity to a calmodulin-like protein from Arabidopsis (D. Bartling, EMBL Data Library, unpublished results). Calmodulins belong to the superfamily of Ca2+-binding EFhand proteins (Moncrief et al., 1990). Within the protein there are two domains encoding typical and two domains encoding similar EF-hand sequences. In Dictyostelium it has been reported that the ribosomal protein L19 has a calmodulin-binding activity (Sonnemann et al., 1991). The finding that a calmodulin antagonist inhibits protein synthesis (Kumar et al., 1991) may point to the physiological role of distinct calmodulins for the regulation of eukaryotic protein synthesis. It can be supposed that the ribosomal protein L19 and calmodulin-like protein are involved in the regulation of protein synthesis during adaptation of tobacco plants to increased assimilate concentrations.