Table 3. Average nucleotide composition (mean% ± SE) of structural components of 16S rRNA genes of 44 mesophiles and thermophiles.

Nucleotide	Mesophile	Thermophile	Significancea
(A) Nucleotide composition of the stems; the average length of stems in a 16S rRNA is 856 nucleotidesb
A	15.7 ± 0.46	7.5 ± 0.59	P < 0.001
C	26.7 ± 0.42	37.0 ± 0.74	P < 0.001
G	34.9 ± 0.45	42.8 ± 0.58	P < 0.001
T	22.7 ± 0.41	12.7 ± 0.73	P < 0.001
(B) Nucleotide composition of the loops (including hairpin loops, multiple branched loops, psudoknot loops and dangling ends); the average length of loops in a 16S rRNA is 427 nucleotidesb
A	37.6 ± 0.35	37.8 ± 0.34	NSc
C	16.5 ± 0.24	17.8 ± 0.21	P = 0.001
G	23.9 ± 0.28	25.1 ± 0.37	P = 0.012
T	21.9 ± 0.26	19.2 ± 0.43	P < 0.001
(C) Nucleotide composition of the internal loops and bulge loops; the average length of internal/bulge loops in a 16S rRNA is 163 nucleotidesb
A	48.2 ± 0.46	45.5 ± 0.42	P = 0.001
C	9.2 ± 0.30	11.3 ± 0.46	P = 0.001
G	25.5 ± 0.59	27.1 ± 0.55	NS
T	17.0 ± 0.56	16.0 ± 0.40	NS

^aProbability of Mann-Whitney U-test of respective nucleotide composition in the two sets of mesophiles and thermophiles.

^bAmbiguous nucleotides were excluded from the calculation.

^cNS, not significant (P > 0.05).