MOLECULAR EVOLUTION OF NITRATE REDUCTASE GENES

To understand the evolutionary mechanisms and relationships of nitrate reductases (NRs), the nucleotide sequences encoding 19 nitrate reduct ase (NR) genes from 16 species of fungi, algae, and higher plants were analyzed. The NR genes examined show substantial sequence similarity, particularly within functional domains, and large variations in GC co ntent at the third codon position and intron number. The intron positi ons were different between the fungi and plants, but conserved within these groups. The overall and nonsynonymous substitution rates among f ungi, algae, and higher plants were estimated to be 4.33 x 10(-10) and 3.29 x 10(-10) substitutions per site per year. The three functional domains of NR genes evolved at about one-third of the rate of the N-te rminal and the two hinge regions connecting the functional domains. Re lative rate tests suggested that the nonsynonymous substitution rates were constant among different lineages, while the overall nucleotide s ubstitution rates varied between some lineages. The phylogenetic trees based on NR genes correspond well with the phylogeny of the organisms determined from systematics and other molecular studies. Based on the nonsynonymous substitution rate, the divergence time of monocots and dicots was estimated to be about 340 Myr when the fungi-plant or algae -higher plant divergence times were used as reference points and 191 M yr when the rice-barley divergence time was used as a reference point. These two estimates are consistent with other estimates of divergence times based on these reference points. The lack of consistency betwee n these two values appears to be due to the uncertainty of the referen ce times.