Abstract:Nitrogen fixation depends on the catalysis of nitrogenase in Nostoc. The nitrogenase iron protein NifH encoded by the highly conserved nifH gene, which is one of the oldest existing functional genes in the history of gene evolution. In this paper, 40 nifH gene sequences of Nostoc and the relative species group of cyanobacteria were selected, and the phylogenetic trees relied on these sequence data were constructed by maximum likelihood method. Then PAML4.9 software was used to analyze the bioinformatics of nifH gene coding proteins in Nostoc. Branch model, site model and branchsite model were used to detect the selection sites of the nifH genes, and to explore the adaptive evolution characters of the nifH genes. Our results indicated that: (1) the phylogenetic tree with the maximum likelihood method showed that the inner group could be divided into six large branches (A, B, C, D, E and F), among which D and E are two big branches, and each big branch contains two special small branches A, F and B, C, among which F branch contains nine Nostoc collected from Gurbantonggute desert in Xinjiang, A branch contains F branch and four Nostoc sphaeroides and B branch contains four Nostoc commune and three Nostoc sp. in this study. The C branch contains five Nostoc and Anabaena sequences downloaded from NCBI database and one Nostoc of this study; (2) in the three evolutionary models analyzed, 14 significant positively selected sites such as 1F, 2S, 3S, 4T, 5A, 6F, 7F, 8I, 9S, 10C, 17I, 27Y, 29D and 31R were detected. Therefore, the results showed that the nifH genes have undergone adaptive evolution in Nostoc. Branchsite model is a better model to study the adaptive evolution of algae genes.