Abstract
I developed a mathematical method to search for DNA regions that are significantly enriched in reverse complement triplets (RCTs) and are located in sequences with strongly expressed triplet periodicity (TP). The method makes it possible to exclude the influence of TP on the number of RCTs. To search for RCTs, I used the difference between triplet frequencies and their expected number, which was determined by taking into account the TP of the analyzed region. I analyzed the genomes of 42 bacteria representing all bacterial phyla, and found that the number of DNA regions containing RCTs ranged from several hundred to several thousand per genome depending on its size. The average length of the region was about 850 DNA bases. The most common inversion symmetry (IS) pattern of the RCT-containing regions was the enrichment of the first, second, and third triplet positions with {A, G}, {A, T}, and {T, C} bases, respectively. When the sequence was rotated 180 degrees and the bases were replaced with complementary ones (IS), such enrichment of triplet positions was preserved. I suggest that the emergence of IS could be a result of evolutionary processes such as inversions, transpositions, and recombinations.