Initiation Factor 2 of Myxococcus xanthus , a Large Version of Prokaryotic Translation Initiation Factor 2

Abstract

We have isolated the structural gene for translation initiation factor IF2 (infB) from the myxobacterium Myxococcus xanthus. The gene (3.22 kb) encodes a 1,070-residue protein showing extensive homology within its G domain and C terminus to the equivalent regions of IF2 from Escherichia coli. The protein cross-reacts with antibodies raised against E. coli IF2 and was able to complement an E. coli infB mutant. The M. xanthus protein is the largest IF2 known to date. This is essentially due to a longer N-terminal region made up of two characteristic domains. The first comprises a 188-amino-acid sequence consisting essentially of alanine, proline, valine, and glutamic acid residues, similar to the APE domain observed in Stigmatella aurantiaca IF2. The second is unique to M. xanthus IF2, is located between the APE sequence and the GTP binding domain, and consists exclusively of glycine, proline, and arginine residues.Myxococcus xanthus is the best-characterized member of the myxobacteria family. These gram-negative soil bacteria are able to undergo a multicellular developmental program in response to starvation. Hundreds of thousands of bacteria glide to aggregation centers to form complex structures known as fruiting bodies. These specialized structures contain differentiated cells, the myxospores (9). During the developmental cycle of M. xanthus, cellular communication involving at least five different extracellular signals, known as A, B, C, D, and E, is required. The D signal corresponds to translational initiation factor 3 (IF3) (6,7,8), which contains a particular C-terminal extension absent in IF3s from other species. A mutation impeding development, mapping within this extension, suggested that the extension is necessary for developmental functions of the cell rather than for viability (8).In prokaryotes, IF3 (encoded by the infC gene) is required for the initiation of translation with at least two other factors, IF1 (encoded by infA) and IF2 (encoded by infB), ribosomes, mRNA, fMet-tRNA f Met , and GTP (13). Our knowledge of this process comes essentially from studies with Escherichia coli. The three factors play essential roles in each step of translation initiation to control the correct entry into the first round of the elongation cycle (22). In E. coli, the infB gene is part of the nusA-infB operon (19,23,24,29). IF2 is an essential GTP binding protein (20) which exists in two major forms in E. coli, IF2␣ (97.3 kDa) and IF2␤ (79.7 kDa). IF2␤ exists in two subforms (␤1 and ␤2), which are barely distinguishable and which are due to two internal in-frame initiation codons separated by only 18 nucleotides (26). The expression of ␣ and ␤ forms of IF2 has also been observed in Bacillus subtilis (31). On the other hand, only one form seems to exist in other bacteria such as Bacillus stearothermophilus, Streptococcus faecium, and the myxobacterium Stigmatella aurantiaca (3, 4, 11). There is thus no obvious pattern to the occurrence of multiple forms of IF2 in gram-negative and gram-positive bacteria or even in ...

Initiation of Protein Synthesis in Bacteria

“…stearothermophilus and T. thermophilus IF2 possess a single domain in the region N-terminal to the G-domain, whereas myxobacterial IF2, which has the longest N-terminal regions characterized in bacteria, is composed of several domains with a highly unusual amino acid composition. The latter is a general feature of the N-terminal regions of IF2 (16,64,225,230). Regions N-terminal to the G-domain of eukaryotic IF2 homologues are generally long, up to ϳ700 amino acids, whereas the regions of the archaeal IF2 homologues are generally short.…”