rRNA processing: removal of only nineteen bases at the gap between 28S alpha and 28S beta rRNAs in Sciara coprophila.

We have determined the sequence of the rDNA region between the 28S alpha and 28S beta rRNA coding segments (termed a "gap") in the insect Sciara coprophila, and have used S1 nuclease mapping and cDNA primer extension to define the 5' and 3' boundaries of the gap. Only 19 bases found in rDNA at the gap region are absent from mature 28S rRNA. Eukaryotic rRNAs contain stretches of nucleotides ("expansion segments") which are absent in E. coli rRNA. The gap region in Sciara is located within expansion segment V. Therefore, the excision of 19 bases in the Sciara gap suggests that a large portion of expansion segment V plays no function in mature ribosomes. Specific sequences conserved in Sciara and Drosophila are considered as candidates for recognition signals for the excision of the gap transcript.