DEVELOPMENTAL AND ENVIRONMENTAL-REGULATION OF 2 RIBOSOMAL-PROTEIN GENES IN TOBACCO

Two cDNA clones, TSC29 and TSC40, were isolated from a cDNA library prepared from three-day-old tobacco cell suspension grown to early exponential stage. DNA sequence analyses and database searches revealed that the TSC29 transcript encodes a protein which is highly homologous to eukaryotic 60S ribosomal (r)-protein L25 and that the TSC40 product is homologous to rat 60S r-protein L34. Southern blot analysis showed that the putative r-protein genes are members of multigene families. Transcript levels of both genes were most abundant in three-day-old cell suspension and declined in older cultures. Transcripts were also present in plant vegetative and reproductive organs. However, for TSC40 in particular, the mRNA levels were lower in plant organs than in three-day-old cell suspension. Stems and roots exhibited higher expression than leaves and flowers, indicating that these clones are differentially regulated in various cell types. Both genes were expressed at low levels in mature seeds but transcript levels significantly increased after one day of germination, remained at a high level until day 4, and declined after day 5. In situ localization experiments with germinating seedlings revealed that the TSC29 transcript was preferentially localized in root tips, epidermis, and endosperm. Wounding increased the steady-state mRNA amounts of these r-protein genes, and 2,4-dichlorophenoxyacetic acid and benzyladenine further increased the transcript level.