Last modified on 30 January 2007.

Table 3: Number of Introns per rRNA Gene

Table 3 presents the number of introns per rRNA gene. While more than 85% of the documented and analyzed rRNA genes do not have an intron, over 1500 16S and 23S rRNAs have at least one intron. Approximately 75% (623) of these genes have a single intron, 15% (127) have two introns, 0.5% (40) have three, 0.25% (20) have four, 0.1% (11) have five, two rRNA genes have 6, 7 or 8 introns, and one rRNA gene has 9 introns.

To determine the amount of bias in the distribution of introns among their exon sequences, the Poisson distribution (here, μ is the mean frequency of introns for a particular exon and x is the target number of introns per rRNA gene) has been used to calculate the number of rRNA sequences expected to contain a given number of introns. Based upon this data, no rRNA sequences are expected to contain four or more introns; in fact, we see a significant number of sequences that contain these large numbers of introns. The observed numbers of sequences exceed the expected values for all but one category: fewer rRNAs contain only one intron than expected.

The two molecules (16S and 23S rRNA) show a differing trend with respect to cell location for those sequences containing large numbers of introns. In 16S rRNA, only nuclear genes (ten) have been observed to contain five or more introns; indeed, of the 57 genes containing three or more introns, only two are not nuclear (both of these are mitochondrial). In 23S rRNA, the trend is both opposite and weaker; of the thirteen rRNA sequences containing four or more introns, five are nuclear (containing five introns), with four chloroplast and four mitochondrial genes comprising the remaining eight sequences.

Table 3 provides access to additional tables, which each present the complete information for every intron within a particular class (e.g., 16S rRNA genes containing two introns), grouped by their exons. As with the other online tables, this information will be updated daily to reflect new intron sequences that are added to this database.