CIS-ACTING ELEMENTS AND EXPRESSION PATTERN OF THE SPINACH RPS22 GENE CODING FOR A PLASTID-SPECIFIC RIBOSOMAL-PROTEIN

In order to study the regulation of nuclear genes coding for plastid r ibosomal proteins, we have analysed the promoter region of spinach rps 22 using both in vitro and in vivo approaches. By footprinting analyse s, we have identified eight DNA elements interacting with spinach leaf nuclear factors in the 300 bp promoter region upstream of the transcr iption start site. Among these elements, four are short AT-rich sequen ces and one is identical to the Hex motif characterized initially in w heat histone genes. In transgenic tobacco plants, the reporter gene co ding for the beta-glucuronidase (GUS) directed by a 1.2 kb upstream re gion of rps22 was expressed in several plant organs, with high levels in leaf mesophyll, embryo cotyledons and root meristematic cells and v ery low levels in other cell types. Interestingly, when deleted to - 2 95, the promoter, which contained all the foot-printed elements, was s till able to confer the same expression pattern, although the activity was relatively lower than with the 1.2 kb promoter. When deleted furt her to - 154, the promoter, from which the AT-rich elements were elimi nated, loses its activity almost completely, suggesting that these AT- rich elements are important for the rps22 promoter activity. Altogethe r, our results show that rps22 gene expression is controlled by specif ic cis elements not present in other nuclear-encoded plastid ribosomal protein genes studied so far.