Location, size, and BHB type of introns in the tRNA genes of 18 Archaea. The schematic cloverleaf sequence corresponds to the consensus sequence of all the tDNAs (257 altogether) from the six archaea that contain introns at positions other than 37/38 (33 altogether; see Table 1). The conventional IUB/IUPAC degenerate DNA alphabet (Cornish-Bowden 1985) is used in this and the following figures: R (purine), A or G; Y (pyrimidine), C or T; S (strong), G or C; B (not A), C, G, or T; D (not C), A, G, or T; H (not G), A, C, or T; V (not T), A, C, or G; N (any), A, C, G, or T. Key to base-pairing consensus is: (plus), Watson–Crick base pairing only; (asterisk), Watson–Crick or G-T/T-G pairings; (number sign), Watson–Crick pairing or mismatch; (minus), Watson–Crick pairing or G-T/T-G pairings or mismatches. Arrows show the positions where introns are located. Their exact locations are also indicated in the first line of each box (for example, 3/4 means that the intron is located between nt 3 and nt 4. Every intron is indicated with the abbreviated name of the species [with those of P. aerophilum (P. aero) highlighted in bold], the tDNA type (amino acid), the length of the intron, and its type of splicing motif (hBHBh′, hBH, or HBh′; see text). The 1/2 and 2/2 indications refer to the six cases of two introns within the same tDNA. In three of these six cases, one of the introns is located at 37/38 and is indicated accordingly. All tDNAs bearing introns in the 18 archaeons examined are essential and single-copy genes, except the case of tDNA-Glu (TTC) of M. kandleri (two copies; see also Table 1).
