Putative Horizontally Acquired Genes, Highly Transcribed duringYersinia pestisFlea Infection, Are Induced by Hyperosmotic Stress and Function in Aromatic Amino Acid Metabolism

Abstract

ABSTRACTWhile alternating between insects and mammals during its life cycle,Yersinia pestis, the flea-transmitted bacterium that causes plague, regulates its gene expression appropriately to adapt to these two physiologically disparate host environments. In fleas competent to transmitY. pestis, low-GC-content genesy3555,y3551,andy3550are highly transcribed, suggesting that these genes have a highly prioritized role in flea infection. Here, we demonstrate thaty3555,y3551,andy3550are transcribed as part of a single polycistronic mRNA comprising they3555, y3554, y3553,y355x, y3551, andy3550genes. Additionally,y355x-y3551-y3550compose another operon, whiley3550can be also transcribed as a monocistronic mRNA. The expression of these genes is induced by hyperosmotic salinity stress, which serves as an explicit environmental stimulus that initiates transcriptional activity from the predictedy3550promoter. Y3555 has homology to pyridoxal 5′-phosphate (PLP)-dependent aromatic aminotransferases, while Y3550 and Y3551 are homologous to the Rid protein superfamily (YjgF/YER057c/UK114) members that forestall damage caused by reactive intermediates formed during PLP-dependent enzymatic activity. We demonstrate thaty3551specifically encodes an archetypal RidA protein with 2-aminoacrylate deaminase activity but Y3550 lacks Rid deaminase function. Heterologous expression ofy3555generates a critical aspartate requirement in aSalmonella entericaaspCmutant, while itsin vitroexpression, and specifically its heterologous coexpression withy3550,enhances the growth rate of anEscherichia coliΔaspCΔtyrBmutant in a defined minimal amino acid-supplemented medium. Our data suggest that they3555, y3551, andy3550genes operate cooperatively to optimize aromatic amino acid metabolism and are induced under conditions of hyperosmotic salinity stress.IMPORTANCEDistinct gene repertoires are expressed duringY. pestisinfection of its flea and mammalian hosts. The functions of many of these genes remain predicted or unknown, necessitating their characterization, as this may provide a better understanding ofY. pestisspecialized biological adaptations to the discrete environments of its two hosts. This study provides functional context to adjacently clustered horizontally acquired genes predominantly expressed in the flea host by deciphering their fundamental processes with regard to (i) transcriptional organization, (ii) transcription activation signals, and (iii) biochemical function. Our data support a role for these genes in osmoadaptation and aromatic amino acid metabolism, highlighting these as preferential processes by whichY. pestisgene expression is modulated during flea infection.