Organisation of the external spacer of nuclear rRNA genes

In maize, rRNA genes are present in thousands of copies leading to a steady supply of rRNAs. The rRNA gene units comprise the genes for the 18S, 5.8S and 25S rRNA in that order, with internal and external spacer regions. The external spacer region is of particular interest since it contains the transcription start for the primary transcript and, presumably, regulatory elements.

To gain insight into the structure of the external spacer we isolated an rRNA gene unit from a gene library of variety A619 and sequenced the spacer region.

The 3020 bp long external spacer contains 9 tandemly arranged 200 bp long repeat units with high homology and only minor insertions or deletions in some of the repeats. Interestingly, the basic repeat structure reveals some homology to the 130 bp repeat unit of the wheat rRNA gene unit isolated by Appels and Dvorak (Theor. Appl. Genet. 63:337-348, 1982). Potential start positions of RNA synthesis as well as the 3' and 5' ends, respectively, of the bordering 18S and 25S rRNA genes were determined by S1 mapping experiments (indicated in the scheme by the arrows).

It will now be interesting to study the influence of the repeat units on the expression of the rRNA gene unit. A comparison of this sequence from variety A619 with that from other varieties, such as the sequence obtained in the lab of J. Rubenstein (University of Minnesota), will furthermore allow the recognition of essential indispensable sequences of the external spacer region.

Figure.

Ch. Toloczyki and G. Feix
 
 


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