A frame-specific symmetry of complementary strands of DNA suggests the existence of genes on the antisense strand

The bacterial DNA sequence in GenBank database were divided into coding and noncoding regions and examined for the base-trimer distribution in every triplet frame on the sense and antisense strands. The results revealed that for the noncoding region, both strands have very similar base-trimer distributions and have no frame specificity; that is, DNA is symmetric in the noncoding region. For the coding region, on the other hand, the symmetry is broken only in the triplet framework, and we found a special triplet-frame-specific symmetry which appears when the two complementary strands of the coding region are read from their 5 ends. In addition, the following frame specificity was also observed in the distribution of stop codons on the antisense strand of the coding region. When the antisense sequences of the open reading frames (ORFs) in the database are read in the three reading frames, the same reading frame as the corresponding ORF contains a significantly larger amount of long open frames without stop codons (i.e., nonstop frames NSFs]) than expected, while the number of NSFs in the other two reading frames is similar to that of the expected one. That is, NSFs as well as ORFs are maintained in a frame-specific manner, and in this sense, DNA becomes symmetrical even in the coding region. These two kinds of frame-specific symmetries indicate that only an ORF and its complementary triplets are specifically recognized and maintained in DNA. We suppose that the antisense strands as well as the sense strands in the coding region may be transcribed, thereby producing various kinds of proteins corresponding to NSFs, though their amount may not be large. The presence of these proteins should have some benefits for living organisms, and therefore we propose that these proteins are upcoming enzymes having novel functions.Correspondence to: I. Urabe