Sugar levels regulate the expression of ribosomal protein genes encoding protein S28 and ubiquitin-fused protein S27a in maize primary root tips

In order to isolate glucose-starvation-repressed cDNAs in maize ( Zea mays L.) root tips, a cDNA library was constructed with poly(A) + mRNA from excised root tips of 3-day old maize seedlings. After differential screening of the library, we isolated two cDNAs named star1 and star2, which were identified by comparison with sequence data bases. star1 corresponds to a new member of the ubiquitin fusion gene family in maize coding for a monomer of ubiquitin in frame with ribosomal protein S27a (Ubi-S27a). star2 encodes the maize homologue of ribosomal protein S28. In maize root tips submitted to carbohydrate deprivation, the Ubi-S27a and S28 mRNA levels decrease very rapidly and become undetectable after 24 h of starvation. The effects of starvation on the Ubi-S27a and S28 mRNA expression can be reversed by re-feeding the root tips with glucose. The Ubi-S27a and S28 mRNA expression is most prevalent when root tips are incubated in the presence of high glucose concentrations and metabolizable sugars such as fructose and sucrose, but not in the presence of non-metabolizable sugars such as mannitol and l-glucose. Transcript levels of maize Ubi-S27a and S28 are preferentially abundant in dividing tissues of maize seedlings. In root tips incubated with cell division inhibitors, the decrease of Ubi-S27a and S28 mRNA steady-state levels parallels that of histone H4 mRNA, that was used as a marker of cell division. These results indicate that the regulation of the expression of the two ribosomal protein genes that we monitored, may be controlled by sugar levels in a nutrient supply- and growth rate-dependent manner.