Ribosomal transcription units integrated via T‐DNA transformation associate with the nucleolus and do not require upstream repeat sequences for activity in Arabidopsis thaliana

Ribosomal repeat units of Arabidopsis thaliana were introduced into the A. thaliana genome via Agrobacterium‐mediated 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.