Xanthomonas caspase displays an inherent PARP-like activity

In an earlier study, intracellular accumulation of metabolites such as pyruvate and citrate in Xanthomonas campestris pv. glycines (Xcg) was found to result in a caspase dependent stationary phase rapid cell death (RCD). In the present study, the presence of poly ADP-ribose polymerase (PARP)-like activity associated with caspase-3-like protein of Xcg is reported. This activity was found to be responsible for depletion of cellular NAD(+) levels in RCD-promoting media such as Luria-Bertani medium and starch medium fortified with citrate. Addition of PARP-specific inhibitors such as 3-aminobenzamide to RCD-promoting media restored the intracellular NAD(+) levels and thereby prevented RCD. The inherent association of PARP-like activity with the caspase protein was demonstrated by PARP cellular assay, immuno-precipitation and Western analysis. A truncated polysaccharide deacetylase gene having a caspase-like domain was cloned. The expressed protein though found to be inactive, cross-reacted with human caspase and PARP antibodies. This is the first report demonstrating the presence of a PARP-like activity in a prokaryote and its involvement in cell death.