The Myxococcus xanthus dsg gene product performs functions of translation initiation factor IF3 in vivo

Abstract

The amino acid sequence of the Dsg protein is 50%o identical to that of translation initiation factor IF3 of Escherichia coli, the product of its infC gene. dsg mutants are able to initiate the developmental process, but their development goes awry when the cells have formed loose asymmetric aggregates, at about 6 to 10 h. These mutants are unable to transcribe a particular set of developmentally regulated genes (3). The dsg mutation also affects the cells during vegetative growth. Mutants form compact, heaped, smooth-edged colonies, unlike the flat, spreading wild-type colonies. These mutants are also defective in the phase variation of colony color. In the accompanying paper (5), we describe the sequence of the dsg gene, which contains a missense mutation in dsg mutants. The amino acid sequence of Dsg is 50% and 51% identical to that of translation initiation factor IF3 of both Escherichia coli and Bacillus stearothermophilus, respectively. We also show that like IF3, Dsg begins with a unique initiation codon and contains conserved residues which are critical for the function of IF3. The Dsg protein extends 66 more amino acids to its C terminus than the IF3 proteins of E. coli, other enteric bacteria, and B. stearothermophilus.An important role of IF3 in translation initiation is to help the ribosome select the initiation codon on the mRNA. Thus, IF3 ensures the fidelity of translation by preventing initiation at an incorrect location in the message or in the wrong reading frame (12,13,26 codon. The IF3 protein, which is associated with the 30S ribosomal subunit during initiation, inspects the codons near the Shine-Dalgarno site for one which will pair correctly with the anticodon on the initiator tRNA, fMet-tRNA et, which is also associated with the 30S subunit. When the proper initiation codon is located, the 50S ribosomal subunit joins the 30S initiation complex to form the 70S ribosome, and the initiation factors are released (8).The infC gene, which encodes IF3 in E. coli, begins with the atypical AUU initiation codon (21). Recent work shows that IF3 selects against initiation at AUU codons and for initiation at AUG, GUG, and UUG codons, the typical initiators. In fact, this atypical AUU initiation codon is involved in the translational autoregulation of infC expression (2,9,26). When IF3 levels are high, the mRNA encoding infC is not translated because IF3 does not recognize AUU for the initiation of translation. However, when IF3 levels are low, the infC message is translated because, in the absence of IF3, the ribosome shows low selectivity for the typical initiation codons. Translation of infC from AUU continues until the levels of IF3 are high enough to restrict translation from an AUU initiation codon, thus regulating the cellular level of the IF3 protein according to its function (1, 2, 9).In this work, we show that Dsg reacts with anti-IF3 serum. Using anti-Dsg antibodies, we show that Dsg is present at constant levels at all times during vegetative growth and development, as would be expected of ...

Unique role for translation initiation factor 3 in the light color regulation of photosynthetic gene expression

“…Interestingly, unlike other bacterial species (29,30), loss of this IF3 is not lethal in F. diplosiphon due to the presence of a second, redundant, highly divergent IF3 that has no apparent role in light regulation. We show that a wide range of bacteria and photosynthetic eukaryotes possess the potential to produce multiple IF3s.…”

“…Measurements were made in both red light (RL) and green light (GL). infCa mRNA levels were normalized by using ribosomal RNA values for each sample (13 signal (Dsg) plays a role in both translation initiation and developmental processes, such as controlling sporulation and fruiting body development (30,39). In the purple nonsulfur bacterium Rhodobacter sphaeroides, the IF3-like protein PifC influences the production of bacteriochlorophyll and photosynthetic complexes (40).…”

Ectopic production of guanosine penta- and tetraphosphate can initiate early developmental gene expression in Myxococcus xanthus.

“…The 2.3-kb E. coli relA-encoding EcoRIBamHI fragment from pALS 10 (a gift from M. Cashel, National Institutes of Health, Bethesda, MD) was inserted into corresponding sites of pMSI27, yielding pMS129. The resulting 2.8-kb HindlII fragment, which now contains the E. coli relA gene behind the M. xanthus carQRS promoter, was inserted into the HindlII site of pPLH343 (Kalman et al 1994), giving rise to pMS132. This plasmid contains the kanamycin resistance gene and the Mx8 phage attachment site, which allows for site-specific recombination within the M. xanthus genome.…”

Molecular Characterisation of the pifC Gene Encoding Translation Initiation Factor 3, which is Required for Normal Photosynthetic Complex Formation in Rhodobacter Sphaeroides NCIB 8253

“…The PifC sequence was compared with the IF3 and Dsg sequences of E. coli and M. xanthus (respective1y)which have both been shown to serve as initiation factors during translation and which themselves exhibit 48.9% identity (66.9% similarity; Fig. 3) (Sacerdot et al, 1982;Cheng et al, 1994;Kalman et al, 1994). Alignment of the R. sphaeroides PifC sequence with E. coli IF3 demonstrated a high level of overall similarity between the two sequences: 60.1 % identity; 78.6% similarity (Fig.…”

“…Buckingham et al, 1990;Phillips-Jones et al, 1993, 199.5). Interestingly, recent studies of Myxucoccm X U I Z I~M S have revealed the importance of a translation initiation factor 3 in the regulation of developmental processes in bacteria Kalman et al, 1994). These studies showed that the Dsg IF3 protein not only performs some of the functions of an IF3 , but also has a specific selective role in fruiting body development and sporulation in this organism .…”