Are guanine nucleotide binding proteins a distinct class of regulatory proteins?

AbstractProteins which bind guanine nucleotides are found in a diverse set of key regulatory positions. They are involved in hormone action, visual transduction, protein synthesis and microtubule assembly. In addition to their ability to bind guanine nucleotides these proteins possess several other common features. (i) They all have similar subunit composition, (ii) they can be ADP-ribosylated, (iii) their conformation changes depending on the nucleotide bound. These regulatory G-proteins have close functional homologies. Do they form a general class of regulatory proteins, like the protein kinases? Do they have a common evolutionary ancestry

Error-prone initiation factor 2 mutations reduce the fitness cost of antibiotic resistance

Mutations in the fmt gene (encoding formyl methionine transferase) that eliminate formylation of initiator tRNA (Met-tRNAi) confer resistance to the novel antibiotic class of peptide deformylase inhibitors (PDFIs) while concomitantly reducing bacterial fitness. Here we show in Salmonella typhimurium that novel mutations in initiation factor 2 (IF2) located outside the initiator tRNA binding domain can partly restore fitness of fmt mutants without loss of antibiotic resistance. Analysis of initiation of protein synthesis in vitro showed that with non-formylated Met-tRNAi IF2 mutants initiated much faster than wild-type IF2, whereas with formylated fMet-tRNAi the initiation rates were similar. Moreover, the increase in initiation rates with Met-tRNAi conferred by IF2 mutations in vitro correlated well with the increase in growth rate conferred by the same mutations in vivo, suggesting that the mutations in IF2 compensate formylation deficiency by increasing the rate of in vivo initiation with Met-tRNAi. IF2 mutants had also a high propensity for erroneous initiation with elongator tRNAs in vitro, which could account for their reduced fitness in vivo in a formylation-proficient strain. More generally, our results suggest that bacterial protein synthesis is mRNA-limited and that compensatory mutations in IF2 could increase the persistence of PDFI-resistant bacteria in clinical settings