Structural organization of the two main rDNA size classes of Ascaris lumbricoides

The two main rDNA size classes in the genome of Ascaris lumbricoides consist of 8.8 kb and 8.4 kb long repeating units present in a quantitative ratio of roughly 10:1. They both contain the genes coding for 18 , 5.8S and 26S ribosomal RNAs. The length heterogeneity is due to a 450 bp long spacer region localized in the longer repeating unit which begins 870 bp upstream of the 5'-end 18S gene. A few additional microheterogeneities in base sequence occur at the 5'-end of the 26S gene. The 18S, 5.8S and 26S coding regions have been mapped on both the 8.8 kb and 8.4 kb repeating units and the localization of the 5'- and 3'-ends of the 18S and 26S genes has been performed by S1 protection. No intervening sequences are present in either coding region of the two main rDNA size classes.