Cycle inhibiting factors (CIFs) are a growing family of functional cyclomodulins present in invertebrate and mammal bacterial pathogens.
Bottom Line: The cycle inhibiting factor (Cif) produced by enteropathogenic and enterohemorrhagic Escherichia coli was the first cyclomodulin to be identified that is injected into host cells via the type III secretion machinery.The X-ray crystal structure of Cif revealed it to be a divergent member of a superfamily of enzymes including cysteine proteases and acetyltransferases that share a conserved catalytic triad.Although these Cif homologs are remarkably divergent in primary sequence, the catalytic triad is strictly conserved and was shown to be crucial for cell cycle arrest, cytoskeleton reorganization and CKIs accumulation.
Affiliation: INRA, UMR1225, Toulouse, France.
The cycle inhibiting factor (Cif) produced by enteropathogenic and enterohemorrhagic Escherichia coli was the first cyclomodulin to be identified that is injected into host cells via the type III secretion machinery. Cif provokes cytopathic effects characterized by G(1) and G(2) cell cycle arrests, accumulation of the cyclin-dependent kinase inhibitors (CKIs) p21(waf1/cip1) and p27(kip1) and formation of actin stress fibres. The X-ray crystal structure of Cif revealed it to be a divergent member of a superfamily of enzymes including cysteine proteases and acetyltransferases that share a conserved catalytic triad. Here we report the discovery and characterization of four Cif homologs encoded by different pathogenic or symbiotic bacteria isolated from vertebrates or invertebrates. Cif homologs from the enterobacteria Yersinia pseudotuberculosis, Photorhabdus luminescens, Photorhabdus asymbiotica and the beta-proteobacterium Burkholderia pseudomallei all induce cytopathic effects identical to those observed with Cif from pathogenic E. coli. Although these Cif homologs are remarkably divergent in primary sequence, the catalytic triad is strictly conserved and was shown to be crucial for cell cycle arrest, cytoskeleton reorganization and CKIs accumulation. These results reveal that Cif proteins form a growing family of cyclomodulins in bacteria that interact with very distinct hosts including insects, nematodes and humans.
Related in: MedlinePlus
Mentions: In E. coli, the cif gene is located on an inducible lambdoid prophage spread widely amongst EPEC and EHEC strains (Fig. 2) . In Photorhabdus strains, cifPl and cifPa are located downstream of a region displaying a high degree of similarity to a prophage described in Serratia entomophila (Fig. 2) . This prophage is integrated 5 to 6 times in the genome of both Photorhabdus species  and encodes genes for several putative virulence factors, notably a putative T3SS effector protein homologous to YopT from Yersiniae. This phage has no homology with the lambdoid prophage found in E. coli isolates but displays some similarity to bacteriocins and R-type pyocins . In B. pseudomallei strain K96243 (Fig. 2), cifBp is located between two vestigial transposase genes on chromosome II near the hrp cluster, which codes for one of the three T3SS present in B. pseudomallei . Comparison of sequenced genomes from different B. pseudomallei strains reveals that the organization of this locus is highly variable. B. pseudomallei strains S13 and 9 contain additional genes, encoding putative transposases, which are inserted near cifBp (Fig. 2). In B. pseudomallei strain 1106a, this region is deleted and cifBp is absent. These data suggest that DNA transposition events could have lead to the heterogeneous distribution of the cifEc-like gene in B. pseudomallei strains. Among the sequenced strains of Y. pseudotuberculosis, only the strain YPIII possesses a gene with similarity to cifEc. Comparison of the genetic environment between YPIII and other Y. pseudotuberculosis strains revealed that cifYp is positioned within a chromosomal locus previously described as the insertion site of ypm, a gene coding for a superantigenic toxin in strain AH . Both ypm and cifYp are located downstream of a 26-bp sequence called yrs which is homologous to dif, a site-specific recombination target used by filamentous bacteriophages for host chromosome integration. Deletions in the yrs locus occur at a higher frequency compared to others regions within the chromosome . Genetic instability at this locus could explain the heterogeneous distribution of both cifYp and ypm genes in the Y. pseudotuberculosis population.