Various regulatory sequences are deprived of their uniqueness by the universal rule of TA/CG deficiency and TG/CT excess

The universal rule of TA/CG deficiency-TG/CT excess endures the extremely high mutation rate of a retrovirus (human immunodeficiency virus type 1) as well as methylation of CAG rather than CG in a plant (maize). Among the consistently abundant nucleotide oligomers, there are two complementary pairs of palindromic nucleotide pentamers containing TG and CA. Out of the CAGTG and CACTG pair emerged the heptameric pair for the long-distance recombination of immunoglobulin genes, CACAGTG and CACTGTG. Reflecting their origin, these heptamers are found everywhere in all DNA, and a substantial fraction of them are accompanied by nonameric components properly spaced from them. It appears that, were the recombination event not confined to B cells, results of illegitimate recombinations might be disastrous. The other pentameric pair is TGCAT and ATGCA. Out of this pair emerged the complementary pair of transcription enhancer decamers: TNATTTGCAT for immunoglobulin light chains and ATGCAAATNA for immunoglobulin heavy chains. Again reflecting their origin, these decamers are found everywhere in all DNA and some genes--for example, in the 3' flanking region of immunoglobulin heavy chain constant region--are accompanied by a downstream "TATA box." It seems that even with regard to the productively recombined immunoglobulin genes, misinitiation of enhanced transcription is a real possibility.