Extremely short 20-33 nucleotide introns are the standard length in Paramecium tetraurelia

Paramecium tetraurelia has the shortest known introns as its standard intron length. Sequenced introns vary between 20 and 33 nucleotides in length. The intron sequences were discovered in genomic sequences coding for a variety of different proteins, including phosphatases, kinases, and low-molecular weight GTP-binding proteins. All intron sequences begin with the conserved dinucleotide GT and end with the conserved dinucleotide AG. The sequences are more AT rich than the Paramecium coding sequences. The identified sequences were confirmed as introns by sequencing several cDNA fragments. We report here analysis of the characteristics of 50 separate introns, including size, base composition, and a consensus sequence.