We have isolated two cDNA clones which appear to encode the 40S ribosomal subunit protein S15a from Brassica napus (oilseed rape). The open reading frame in both clones contains 390 bases, encoding a deduced polypeptide sequence of 130 amino acids (100% homology between clones) with 76% sequence identity to the N-terminal 37 amino acids of the rat ribosomal protein S15a and 80% identity to the S24 polypeptide of yeast. Both the yeast and rapeseed proteins have a net positive charge of +9 and the rapeseed S15a protein has a molecular mass of 14778 Da compared to 14762 Da for the yeast protein. The rapeseed ribosomal protein S15a is encoded by a small multi-gene family with at least two actively transcribed members. A single transcript of ca. 1.0 kb, corresponding to ribosomal protein S15a, is abundant in actively dividing tissues such as apical meristem, flower buds and young leaves and less abundant in mature stem and fully expanded leaves.

Cytokinins play an important role in plant development. We investigated the possibility that the nopaline Ti plasmid gene (tzs) from Agrobacterium tumefaciens could encode a protein able to participate in plant cytokinin production and lead to alterations in plant phenotype as a result of the expression of endogenous tzs. tzs was placed under the control of a heat‐inducible promoter from the Zea mays hsp70 gene. The expression of this fused gene was examined in transgenic Brassica napus plants. The tzs gene, which encodes the enzyme dimethylallyl transferase, was used as a cytokinin biosynthetic gene. The expression of the tzs gene was monitored by RNA hybridization and analysis of cytokinin content. Overproduction of cytokinin was observed even when the plants had not been heat‐shocked, and the plants displayed a reduced root system, increased height and branching, and delayed flowering. In addition, a significant increase in seed yield was observed in the transgenic plants, accounted for by increased number of seeds per silique and seed weight. The results suggest that increased levels of cytokinins, through the expression of tzs, are correlated with growth rather than with differentiation processes.

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