Abstract

Genomic islands (GIs) are horizontally acquired DNA regions which carry genes encoding advantageous traits for the host that harbors them. Recently identified, the Enterobacteriaceae-associated ROD21-like family of GIs comprise more than 50 different islands present among several strains of the family Enterobacteriaceae, including plant and animal pathogens. The most studied member of this family is the pathogenicity island ROD21, which is harbored by Salmonella enterica serovar Enteritidis, a foodborne pathogen that can produce gastroenteritis and a severe invasive infection in susceptible individuals. Research from our laboratory has found that the excision of ROD21 plays a role in the pathogenesis of Salmonella serovar Enteritidis, however, the mechanism that regulates this process remains unknown. Aiming to understand the regulation of the excision of ROD21, we sought to characterize the gene SEN1998, which is thought to be directly involved in this process. For this, we analyzed the predicted product SEN1998 comparing it with known RDFs through multiple sequence alignment and prediction of the secondary and tridimensional structures. At the same time, the excision of ROD21 and the expression of its integrase-encoding gene SEN1970 were quantified in the wildtype, ΔSEN1998 and complemented strains of Salmonella serovar Enteritidis phage type 4. We found that SEN1998 has the main features of RDFs, including conserved key amino acids and secondary-structure elements, and a putative winged-helix DNA-binding domain. Nevertheless, the absence and the overexpression of SEN1998 had only a slight effect on the excision, which became statistically significant when the mutant and complemented strains were compared. A further assessment of the excision and expression along the growth curve in the wildtype strain showed that both the excision and the expression of SEN1970 and SEN1998 have a similar pattern: a minimal value at the logarithmic phase followed by an increase during the transition to the stationary phase, where a plateau is reached. Together, our results suggest that SEN1998 encodes an RDF but, contrary to what happens with bacteriophages or some other GIs, it requires another factor to induce the excision. We hypothesize that SEN1998 and the integrase SEN1970 need to be co-expressed to induce the excision of ROD21.