In Reply: Does Diphtheria Toxin Have Nuclease Activity?

Mechanistic aspects of the deoxyribonuclease activity of diphtheria toxin

Here we examined the intrinsic nuclease activity of diphtheria toxin (DTx) to determine the mechanism by which it catalyzes DNA degradation. Results show that DTx degrades double-stranded DNA (dsDNA) by non-processive, endonucleolytic attack, without apparent specificity for nucleotide sequence. Moreover, divalent cation composition determines whether supercoiled dsDNA is cleaved by the introduction of single-strand nicks or double-strand breaks. Circular single-stranded DNA (ssDNA) is also a substrate for endonucleolytic attack. Pre-incubation of DTx with a 2000-fold excess of NAD, the natural substrate for the toxin's ADP-ribosyltransferase (ADPrT) activity, inhibited the transfer of radiolabeled ADP-ribose to elongation factor 2 but had no effect on the degradation of radiolabeled DNA. Based on this result and the fact that compounds known to inhibit the ADPrT activity of DTx had no effect on its nuclease activity and pre-incubation of DTx with DNA had no effect on ADPrT activity, we conclude that the ADPrT and nuclease active sites of DTx are functionally and spatially distinct. Moreover, studies with an ADPrT-inactivated form of DTx indicate that nuclease activity alone can lead to target cell lysis.

The pathology of diphtheria

Diphtheria is an acute, communicable disease caused by Corynebacterium diphtheriae. The disease is generally characterized by local growth of the bacterium in the pharynx with pseudomembrane formation or, less commonly, in the stomach or lungs; systemic dissemination of toxin then invokes lesions in distant organs. Acute disease of the upper respiratory tract usually involves one or more of the following: tonsillar zones, larynx, soft palate, uvula, and nasal cavities. A recent epidemic in Russia emphasized the role of vaccination in reducing disease in children and adults.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

Localization of diphtheria toxin nuclease activity to fragment A

We describe a series of experiments that aimed to establish whether nuclease activity is actually associated with diphtheria toxin (DTx) and its A subunit (DTA), as we originally reported (M. P. Chang, R. L. Baldwin, C. Bruce, and B. J. Wisnieski, Science 246:1165-1168, 1989). Here we show that (i) trypsinization of DTx does indeed produce nucleolytically active DTA, (ii) reduction of electroeluted, unreduced, cleaved DTx (58 kDa) yields nuclease-active DTA (24 kDa), and (iii) fractionation of DTx and DTA by anion-exchange chromatography leads to coelution of nuclease activity with both forms of the toxin, even though each form elutes at a distinct salt concentration. In addition, we show that Escherichia coli-derived DTA also expresses nuclease activity. These studies confirm our initial assertion that the nuclease activity observed in DTx preparations is intrinsic to the DTA portion of DTx.

Minireview: enzymatic properties of ribosome-inactivating proteins (RIPs) and related toxins

Ribosome-inactivating proteins (RIPs) are a group of proteins that inhibit protein synthesis in eucaryotic cells. While the biological effects have been well characterized, the underlying enzymatic mechanisms have not been elucidated until recently. Two different mechanisms have been identified. Plant and bacterial RIPs act as N-glycosidases. They cleave a single N-glycosidic bond between adenine and ribose at a specific nucleotide A-4324 of the 28S rRNA of the 60S ribosomal subunit. On the other hand, the fungal RIPs act as ribonucleases and cleave a single phosphodiester bond between G-4325 and A-4326 of the same rRNA, just one nucleotide away from the site of action of plant/bacterial RIPs. Other protein synthesis inhibitory proteins act by their ADP-ribosyltransferase activity which modify and thus inactivate elongation factor-2. Recently, some toxins have been shown to possess deoxyribonuclease activity which may also account for their toxicity.