Carocin S2 degraded 5′-labeled total RNA but not 5′-labeled CaroS2K-free RNA (Figure 8B), and the amount of degradation was not dose-dependent (arrowhead). However, the appearance of segments of unknown origin paralleled partial degradation of 23S and 16S rRNA (Figure 8C). These results suggest that the site of excision (either conformational or sequential) is close to the 5′-terminus of rRNA. Notably, the decrease in the amount of rRNA depended on the amount of Carocin S2 protein present, with complete degradation occurring in the presence of excess Carocin S2. Ogawa et al. reported that RNase type of bacteriocins, colicin E3 and colicin E5, catalyze
the hydrolysis of the shorter RNAs from 16S rRNA [19, 32]. Moreover, colicin E5 was found Cediranib purchase to hydrolyze tRNA in vitro. Furthermore, it was previously reported that colicin E3 cleaved 16S rRNA completely, and even 30S rRNA [11, 33]. In our study, carocin S2 acted as an RNase that hydrolyzes rRNA (both 23S and 16S) in vitro. In terms of enzymatic function, Carocin S2 may act as an endo- and exo-ribonuclease simultaneously. Moreover, CaroS2I
significantly inhibited nuclease activity in vitro but not in vivo (Figures 7, Figure 8 andAdditional file 1, Figure S3). We speculated that immunity protein CaroS2I might not be able to cross the cell membrane, as previously described . Although our in vitro experiment showed that carocin S2 was a ribonuclease, further investigation is needed to clarify its Ganetespib function Carbohydrate in cells. One of the other Tn5 insertional mutants, TF1-1, which disrupted the coding sequence of the fliC gene, was found to Baf-A1 mouse halt expression of Carocin S2 (Figure 1), indicating that Carocin S2 can also be secreted via the type III secretion system . The role of carocin S2 as an RNase in the cytoplasm is to prevent protein synthesis by cleaving either 23S rRNA or 16S rRNA. The role of the immunity protein, CaroS2I, is usually to stop the damage caused by CaroS2K
in the cytoplasm. More details of the actual mechanism of carocin S2 remain to be elucidated. Conclusion As shown herein, the novel bacteriocin, Carocin S2, was characterized as a ribonuclease. It is the first bacteriocin with ribonuclease activity to be found in Pectobacterium strains. We suggested that Carocin S2 kills the indicator cell by exhausting its supply of some kinds of RNA, leading to inactivation of protein biosynthesis. It will be of interest to study the proteomics of Carocin S2 and its mechanism of action in the future. Methods Bacterial strains, media, and growth conditions Bacterial strains and plasmids used in the study are listed in Table 1. Isolates of Pcc were grown at 28°C in Luria-Bertani (LB) medium or IFO-802 medium. The IFO-802 medium was supplemented with 1% polypeptin, 0.2% yeast extract, 0.1% MgSO4 (pH 7.0), and 1.5% agar. Isolates of Pcc were distinguished from Escherichia coli by their ability to grow on Modified Drigalski’s agar medium .