Table 1.

Statistical Significance for the Nonrandom Orientation of IS Elements

	Proteobacteria			Actinobacteria			Firmicutes
	Chr.	IS	Orient.	Chr.	IS	Orient.	Chr.	IS	Orient.
IS1	33	1144	0.295	—	—	—	—	—	—
IS3	358	2601	0.273	100	968	0.380	159	1115	5.95 × 10−4
IS4a	43	251	0.220	18	83	0.260	—	—	—
IS4b	17	82	0.116	—	—	—	8	21	0.511
IS4c	10	84	0.0903	—	—	—	—	—	—
IS5a	135	735	0.749	—	—	—	83	439	5.00 × 10−6
IS5b	175	969	0.336	55	344	0.360	8	27	0.375
IS5c	51	192	0.684	—	—	—	—	—	—
IS5d	—	—	—	24	100	0.466	7	47	0.233
IS6	50	153	0.367	9	42	0.815	35	104	1.21 × 10−3
IS21	203	787	0.0859	68	218	0.0183	161	509	0.380
IS30	80	289	0.241	53	186	0.200	91	500	6.00 × 10−6
IS66a	135	640	0.0876	—	—	—	33	97	0.0193
IS66b	13	44	0.849	4	36	0.811	7	26	0.667
IS91	63	174	6.53 × 10−3	—	—	—	11	26	0.138
IS110	260	1442	6.10 × 10−5	73	439	0.345	112	721	<1 × 10−6
IS200	221	651	<1 × 10−6	—	—	—	89	270	<1 × 10−6
IS200/IS605	27	58	0.0901	11	23	0.399	38	90	9.40 × 10−5
IS256	152	711	0.0140	79	578	0.510	97	564	0.0161
IS481	21	260	0.0357	43	147	0.493	—	—	—
IS607	—	—	—	8	30	1.00 × 10−6	28	103	<1 × 10−6
IS630	145	1061	0.0398	39	252	0.379	25	120	0.565
IS701	47	234	0.223	33	168	0.118	6	26	0.204
IS982	30	285	0.813	—	—	—	24	145	0.653
IS1182	86	291	0.0545	26	82	0.294	79	386	0.312
IS1380	28	139	0.204	16	39	0.339	15	73	0.673
IS1595	95	333	0.340	6	26	0.157	7	33	0.519
IS1634	8	49	0.227	—	—		15	95	0.282
ISAs1	48	201	0.0142	14	249	0.756	8	54	0.0138
ISL3	91	336	0.343	56	333	0.270	84	496	<1 × 10−6
ISNCYa	68	221	0.139	—	—	—	53	203	<1 × 10−6
ISNCYb	—	—		10	72	9.69 × 10−3	—	—	—
Tn3	70	138	0.0186	20	43	0.237	—	—	—
Tn7	46	61	0.0875	—	—	—	—	—	—

Note.—The table presents, for the groups Proteobacteria, Actinobacteria, and Firmicutes, the number of chromosomes in which a particular IS family was detected, the total number of copies, and the orientation test (Orient.), with P values representing the probability of obtaining, in 10⁶ sample Monte Carlo simulations, a value as extreme as the one that was calculated from the observed data (see Tests on the orientation distribution of IS elements at Phylum level under Materials and Methods) are presented in the table. See supplementary table S2, Supplementary Material online, for nomenclature and IS classification scheme. The analyzed genomes are those marked in red in supplementary table S3, Supplementary Material online (one strain per species to avoid database redundancy). P values (P < 0.05 in bold, P < 10⁻² shaded gray) indicate an asymmetric distribution of IS elements in chromosomes in terms of their orientation relative to the local GC skew sign. IS families with less than 20 copies detected were omitted.