SummaryRibosomal repeat units of Arabidopsis thaliana were introduced into the A. thaliana genome via Agrobacteriurnmediated transformation. Ribosomal transgenes integrated into chromosomal regions outside the nucleolus organizers. Cytological data suggest that the transgenes associate with a nucleolus. To allow detection of transgenic rRNA, a short extension was inserted into the V1 variable region of the 25S ribosomal gene. The RNA transcript from the transgene undergoes a series of maturation steps, including correct processing of the 5' end of 25S rRNA. Using primer extension analysis, expression of a complete rDNA repeat unit was compared with the activity of a repeat unit lacking a sequence called "upstream Sal repeats'. No qualitative or quantitative differences were detected, suggesting that upstream repeat sequences of the rDNA intergenic region do not act as transcriptional enhancers for RNA polymerase I in A. thaliana.

The sequence containing 'upstream Sal repeats' (USR) from the Arabidopsis thaliana ribosomal DNA intergenic region (IGR) was tested for its influence on the in vivo activity of nearby protein coding genes. On average, the presence of the IGR fragment leads to a four-fold increase in the expression of a reporter gene, beta-glucuronidase, under control of the strong CaMV 35S promoter. With the help of the site-specific cre-lox recombination system, we have also obtained pairs of transgenic lines with or without the USR-containing fragment, both integrated at the same chromosomal position. Results with these transgenic lines, which contain an NPT II (kanamycin resistance) gene under control of the nos promoter as a test gene, confirmed the results obtained with the CaMV 35S-driven GUS gene. Moreover, they show that the IGR sequence can oppose tendencies of gene silencing. We hypothesize that the described effect relates to features of the chromatin structure in the proximity of the upstream Sal repeats.