show Abstracthide AbstractRNA degradation is a crucial process in bacterial cells for maintaining proper transcriptome homeostasis and coping with changing environments. A specialized ribonuclease known as RNase J (RnJ) participates in mRNA turnover in many Gram-positive bacteria; however, nothing is known about this process in Corynebacterium diphtheriae, nor is the identity of this RNase. We report here that C. diphtheriae DIP1463 encodes a predicted RnJ homolog, comprised of an N-terminal beta-lactamase domain, followed by beta-CASP and C-terminal domains. We show that a recombinant protein encompassing the beta-lactamase domain possessed 5'-exoribonuclease activity, which was abolished by alanine-substitution of conserved catalytic residues His186 and His188. Intriguingly, deletion of DIP1463/rnj in C. diphtheriae caused slow growth and augmented cell width. Comparative RNA-seq analysis revealed that RnJ controls a large regulon encoding many factors predicted to be involved in biosynthesis, regulation, transport, and iron acquisition. One up-regulated gene in ?rnj mutant is ftsH, coding for the cell division protein FtsH, an inner membrane protease. Interestingly, overexpression of FtsH in the wild-type strain also caused cell-width augmentation. However, unlike the rnj mutant, which was attenuated in a Caenorhabditis elegans model of infection, the FtsH-overexpressing strain exhibited the same virulence phenotype as the wild-type strain. Remarkably, under iron-depleted conditions, production of the exotoxin diphtheria toxin was significantly reduced in the rnj mutant compared to the wild-type strain, likely due to dysregulated secretion of the toxin. Evidently, RNase J is a key ribonuclease that post-transcriptionally influences the expression of factors vital to C. diphtheriae physiology and virulence. Overall design: Examination of RNase J deficient mutant compared to wildtype in biological triplicates