An insight into the evolution of introns in the gyrase A gene of plants

DNA gyrase is a type II topoisomerase essential for replication and transcription in prokaryotes and eukaryotic cell organelles. The functional gyrase enzyme is an A 2 B 2 tetramer encoded by the gyrA and gyrB genes. Most of the eukaryotic gyrase A genes possess introns while they are intron-less in prokaryotes. In the present study, we found out the evolutionary passage of intron development in gyrase A gene with the help of bioinformatics approaches. All the plant gyrase A genes studied by us were found to be a part of the nuclear genome, and their respective proteins were targeted to the organelles. Except the green alga Bathycoccus prasinos , these genes contained introns, and the positions of the homologous introns were found to be highly conserved in diverse plant lineages despite having variation in their nucleotide sequence compositions and lengths. However, in red, brown, and green algae: Chlorella variabilis and Chlamydomonas reinhardtii , homologous intron positions were not conserved, which might be due to the independent acquisition of introns. The study makes it amply evident that the introns appeared in the gene following endosymbiotic gene transfer of the gyrase A to the nuclear genome of an ancestral green plant. The land plants appear to have acquired intron-bearing gyrase A gene from a common ancestral green algae and subsequently lesser re-arrangement of introns at homologous positions resulted in their positional conservation. However, the introns which are known to be under lesser selection pressure evolved differently in various plant species in terms of base composition and lengths.