One Gene and Two Proteins: a Leaderless mRNA Supports the Translation of a Shorter Form of the Shigella VirF Regulator.
VirF, an AraC-like activator, is required to trigger a regulatory cascade that initiates the invasive program of Shigella spp., the etiological agents of bacillary dysentery in humans. VirF expression is activated upon entry into the host and depends on many environmental signals. Here, we show that the virF mRNA is translated into two proteins, the major form, VirF30(30 kDa), and the shorter VirF21(21 kDa), lacking the N-terminal segment. By site-specific mutagenesis and toeprint analysis, we identified the translation start sites of VirF30and VirF21and showed that the two different forms of VirF arise from differential translation. Interestingly, in vitro and in vivo translation experiments showed that VirF21is also translated from a leaderless mRNA (llmRNA) whose 5' end is at position +309/+310, only 1 or 2 nucleotides upstream of the ATG84 start codon of VirF21The llmRNA is transcribed from a gene-internal promoter, which we identified here. Functional analysis revealed that while VirF30is responsible for activation of the virulence system, VirF21negatively autoregulates virF expression itself. Since VirF21modulates the intracellular VirF levels, this suggests that transcription of the llmRNA might occur when the onset of the virulence program is not required. We speculate that environmental cues, like stress conditions, may promote changes in virF mRNA transcription and preferential translation of llmRNA.



Shigella spp. are a major cause of dysentery in humans. In bacteria of this genus, the activation of the invasive program involves a multitude of signals that act on all layers of the gene regulatory hierarchy. By controlling the essential genes for host cell invasion, VirF is the key regulator of the switch from the noninvasive to the invasive phenotype. Here, we show that the Shigella virF gene encodes two proteins of different sizes, VirF30and VirF21, that are functionally distinct. The major form, VirF30, activates the genes necessary for virulence, whereas the minor VirF21, which shares the C-terminal two-thirds of VirF30, negatively autoregulates virF expression itself. VirF21is transcribed from a newly identified gene-internal promoter and, moreover, is translated from an unusual leaderless mRNA. The identification of a new player in regulation adds complexity to the regulation of the Shigella invasive process and may help development of new therapies for shigellosis.
Istituto Pasteur Italia-Fondazione Cenci Bolognetti, Department of Biology and Biotechnology C. Darwin, Sapienza Università di Roma, Rome, Italy.
Maria Letizia
ML
Di Martino
Department of Cell and Molecular Biology, Biomedical Center, Uppsala University, Uppsala, Sweden.
Cédric
C
Romilly
Department of Cell and Molecular Biology, Biomedical Center, Uppsala University, Uppsala, Sweden gianni.prosseda@uniroma1.it gerhart.wagner@icm.uu.se.
E Gerhart H
EG
Wagner
Istituto Pasteur Italia-Fondazione Cenci Bolognetti, Department of Biology and Biotechnology C. Darwin, Sapienza Università di Roma, Rome, Italy.
Bianca
B
Colonna
Istituto Pasteur Italia-Fondazione Cenci Bolognetti, Department of Biology and Biotechnology C. Darwin, Sapienza Università di Roma, Rome, Italy gianni.prosseda@uniroma1.it gerhart.wagner@icm.uu.se.
Gianni
G
Prosseda
D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D004268:DNA-Binding Proteins; D015964:Gene Expression Regulation, Bacterial; D006801:Humans; D016297:Mutagenesis, Site-Directed; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics*; D012763:Shigella flexneri / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic; D014774:Virulence
D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D012333:RNA, Messenger; D014157:Transcription Factors