Difference between two major size classes of carrot rDNA repeating units is due to reiteration of sequences of about 460 bp in the large spacer.

Based on the previous observation that the different regions between two major size classes of carrot rRNA genes are located in their large spacer, detailed physical maps of the spacer region were constructed by cloning, followed by restriction analysis. As a result, the different regions were restricted to BamHI segments of 1.3 kb and 1.8 kb. Sequence analysis of these segments revealed that the shorter one carried one truncated and two complete copies of about 460 bp of repetitious sequences, while the longer one contained one truncated and three full copies of the repetitious sequences. S1 nuclease mapping data suggest that either transcription initiation sites or processing sites of precursor rRNA are located in the repetitious sequence closest to the 18 S rRNA coding region.