Protein-protein interaction (PPI) networks are the backbone of all processes in living cells. In this work we investigate how gene conservation, essentiality and functional repertoire are reflected in the connectivity k of the corresponding proteins in the PPI networks  of 42 evolutionarily unrelated bacterial species . We investigate three issues: i) the degree distribution and densities of the PPI subnetworks of essential and nonessential genes; ii) how the conservationof genes , measured both by the evolutionary retention index (ERI) and by evolutionary pressure (as represented by the ratio K_a/K_s  ) is related to the the degree of the corresponding protein in the PPI network ; iii) how k of the PPI networks selects functional repertoires , as represented by the Clusters of Orthologous Proteins (COGs). We show that conservation, essentiality and functional specialisation of the genes control in a  universal way the topology of the emerging bacterial PPI networks. Noteworthy, a phase transition is observed such that, for  k\ge40, bacterial PPI network are mostly populated by genes that are conserved, essential and which, in most cases, belong to the COG J, a category of genes which is related to ribosomal functions and to the processing of genetic information.
Protein-protein interaction (PPI) networks are the backbone of all processes in living cells. In this work we relate conservation, essentiality and functional repertoire of a gene to the connectivity k of the corresponding protein in the PPI networks . Focusing on a set of 42 bacterial species with reasonably separated evolutionary trajectories, we investigate three issues: i) whether the distribution of connectivity values changes between PPI subnetworks of essential and nonessential genes; ii) how gene conservation , measured both by the evolutionary retention index (ERI) and by evolutionary pressures (evaluated through the ratio K_a/K_s and ENC plots ) is related to the the connectivity of the corresponding protein ; iii) how PPI connectivities are modulated by evolutionary and functional relationships , as represented by the Clusters of Orthologous Proteins (COGs). We show that conservation, essentiality and functional specialization of genes control in a quite universal way the topology of the emerging bacterial PPI networks. Noteworthy, a structural transition in the network is observed such that, for connectivities k\ge40, bacterial PPI networks are mostly populated by genes that are conserved, essential and which, in most cases, belong to the COG cluster J, related to ribosomal functions and to the processing of genetic information.
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