Hydrogen bonding and packing density are factors most strongly connected to limiting sites of high flexibility in the 16S rRNA in the 30S ribosome

Table 2 Linear regression models and statistics for estimating crosslinking frequencies

a) Estimation of UV crosslinking frequencies from internucleotide geometry¹
Model:Freq = β ₁ (1/(C1'-C1'))			Statistics ²
Data – Variables	β₁		Adj. R²	p-value
1a – 1/(C1'-C1')	2.66		0.69	5.6 × 10^-6
1b – 1/(C1'-C1')	2.58		0.64	2.2 × 10^-5
1c – 1/(C1'-C1')	2.80		0.60	5.6 × 10^-5
Model:Freq. = β ₁ (1/(C1'-C1'))(COS(BPA)) + β ₂ (1/(C1'-C1'))(SIN(BPA))
Data – Variables	β₁	β₂	Adj. R²	p-value
2a – 1/(C1'-C1'), BPA	1.4	3.0	0.69	3.4 × 10^-5
2b – 1/(C1'-C1'), BPA	2.9	2.4	0.66	7.0 × 10^-5
2c – 1/(C1'-C1'), BPA	2.9	1.2	0.61	1.9 × 10^-4
b) Estimation of UVA-s⁴U crosslinking frequencies from internucleotide geometry¹
Model:Freq = β ₁ (1/(C1'-C1'))
Data – Variables	β₁		Adj. R²	p-value
3a – 1/(C1'-C1')	11.2		0.57	7.2 × 10^-5
3b – 1/(C1'-C1')	10.6		0.54	1.3 × 10^-4
3c – 1/(C1'-C1')	12.0		0.62	7.2 × 10^-6
Model:Freq. = β ₁ (1/(C1'-C1'))(COS(BPA)) + β ₂ (1/(C1'-C1'))(SIN(BPA))
Data – Variables	β₁	β₂	Adj. R²	p-value
4a – 1/(C1'-C1'), BPA	11.9	2.6	0.54	5.3 × 10^-4
4b – 1/(C1'-C1'), BPA	13.2	1.4	0.58	2.4 × 10^-4
4c – 1/(C1'-C1'), BPA	8.4	11.4	0.61	4.8 × 10^-5

¹ In each case, the geometrical parameters were calculated from the (a) E. coli I, (b) E. coli II [13] and (c) T. thermophilus [11] 30S structures. β₁ and β₂ are the constants determined in the linear regression analysis. C1'-C1' is the distance between C1' atoms of the nucleotides in the observed photocrosslinks. BPA is base plane angle, the torsion angle between the planes of the bases of nucleotides observed in crosslinks, measured in degrees.
² The Adj. R² values indicate the fraction of the variance of the frequency that is accounted for by the regression function. The p values are the probabilities that the frequency is not correlated to the geometry parameter(s).

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