Last modified on 30 January 2007.

Table 2: Intron Type

THIS TEXT IS NEARLY USELESS -- the table has changed too much.

The introns are organized by intron type, as defined above, in Table 2. The frequency of 16S and 23S rRNA exons, non-rRNA exons, number of intron positions in the 16S and 23S rRNA, cell locations, and number of phylogenetic groups for each intron type are tabulated. The highlights of this table are: 1) Of the 1184 known rRNA introns, 980 (83%) are group I, 21 (2%) are group II introns, and the remaining 183 (15%) are unclassified (see below). While only 2% of the rRNA introns are group II, 62% (728/1180) of the non-rRNA introns are group II. In addition to the group II introns, nearly all of the IC3 introns do not occur in rRNAs. 2) The majority of the rRNA group I introns (851/980 = 87%) fall into one of three subgroups: I (276 introns), IC1 (415 introns), and IE (160 introns). 3) As noted earlier, there are three times as many 16S rRNA group I introns than 23S rRNA group I introns (753 vs. 227). 4) Among the three cellular organelles in eucaryotes, 1010 introns (85%) occur in the nucleus, 133 (11%) in the mitochondria, and 41 (4%) in the chloroplasts. 5) The subgroups IC1, IC3 and IE are only present in the nucleus, while the IA, IB, IC2, ID, and II subgroups occur almost exclusively in chloroplasts and/or mitochondria.

The 183 introns described in Table 2 as "Unclassified" merit special attention. All of these introns do not fall into either the group I and group II categories; however, two notable groups of introns are included within the "Unclassified" category. The first is a series of 43 introns occurring in Archaeal rRNAs (the Archaeal introns). Thirty-one of the known Archaeal introns are found in 16S rRNA and the remaining twelve are from 23S rRNA exons. The Archaeal introns range in length from 24 to 764 nucleotides, with an average length of 327 nucleotides. The second group contains 121 spliceosomal introns found in fungal rRNAs. 92 spliceosomal introns are from 16S rRNA and 29 are from 23S rRNA; the lengths of these introns range from 49 to 292 nucleotides. A future version of this database will include both of these groups as separate, distinct entries. Both the Archaeal and splicesomal introns occur only in nuclear rRNA genes and tend to occur at unique sites; the lone exception is the spliceosomal intron from Dibaeis baeomyces nuclear 23S rRNA position 787, a position where a group IIB intron occurs in mitochondrial Marchantia polymorpha rRNA. The Unclassified group contains 21 introns that do not fall into any of the four previously discussed categories (group I, group II, Archaeal, or spliceosomal), including all four mitochondrial introns in this group.

Table 2 expands the presentation by providing links to twenty additional tables, each of which provides expanded information about a specific intron type. The organism name, exon, intron position, cell location, and complete phylogeny are accessible for each intron from these tables. These online tables are dynamically updated daily as information about new introns is made available.