# Nisin E, a New Nisin Variant Produced by _Streptococcus equinus_ MDC1
## 4 Discussion  
Bacteriocin production is a common occurrence among the streptococci, presumably as an anti-competitor mechanism, and a wide variety of bacteriocin types have been characterized from nearly every _Streptococcus_ species. While some species, for example _Streptococcus salivarius_, produce a myriad of antibacterial agents including non-ribosomally \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & & \multicolumn{3}{c}{**Homology Scores**} \\ \hline
**Protein (Gene) 1** & **Putative Function** & \begin{tabular}{c} **Length in Amino Acids** \\ **(Nsu Counterpart)** \\ \end{tabular} & \begin{tabular}{c} **\% Identity 2** \\ \end{tabular} & \
\begin{tabular}{c} **\% Similarity** \\ \end{tabular} \\ \hline NmdP (_nmdP_) &
\begin{tabular}{c} Serine protease (removal of NmdA \\ leader peptide) \\ \end{tabular} & 455 (461) & 58.9 & 75.3 \\ NmdR (_nmtdR_) & Response regulator (regulation of synthesis) & 227 (232) & 76.3 & 88.8 \\ NmdK (_nmtdK_) & Histidine kinase (regulation of synthesis & 446 (448) & 56.3 & 75.1 \\ NmdA (_nmdA_) & Nisin E precursor peptide & 56 (55) & 83.9 & 92.9 \\ Nisin E & Nisin E propeptide (active peptide) & 32 (31) & 90.6 & 93.8 \\ NmdF (_nmdF_) & Immunity protein & 230 (230) & 74.8 & 85.2 \\ NmdE (_nmdE_) & Immunity protein & 244 (253) & 61.3 & 76.3 \\ NmdG (_nmdG_) & Immunity protein & 216 (230) & 51.7 & 70.0 \\ NmdB (_nmtdB_) & Modification protein (dehydratase) & 993 (991) & 56.7 & 76.1 \\ NmdT (_nmdT_) & Transport protein (bacteriocin export) & 598 (598) & 66.1 & 83.1 \\ NmdC (_nmdC_) & Modification protein (thoither formation) & 420 (425) & 52.1 & 71.3 \\ NmdI (_nmdl_) & Immunity protein & 245 (238) & 47.4 & 68.4 \\ \hline \hline \end{tabular}  
* The genes are listed in order of their location in the genetic locus (5′ \(\rightarrow\) 3′). 2 Homology scores were determined with sequence gaps taken into account.  
\end{table} 
Table 3: Proteins encoded by the _nmd_ (nisin E) locus and comparison with its Nsu (nisin U) counterparts. Figure 2: Predicted structures of the mature nisin U (adapted with permission from Ref. [10], 2006, [PERSON]) and nisin E peptides. The post-translationally modified amino acids are highlighted in blue text, and the amino acid differences between nisin U and nisin E are highlighted in red text. Dha, 2,3-didehydrodaalanine; Dhb, 2,3-didehydrobutyrate; Lan, lanthionine; Mln, \(\beta\)-methyl-lanthionine. synthesized peptides [22], other species have only been shown to produce a single type, e.g., _Streptococcus dysplasia_ and _Streptococcus agalactiae_[8; 23]. _Streptococcus equinus_ may be in the latter category as only one other bacteriocin, namely bovicin HC5 (a member of the streptin class of lantibiotics), has been characterized to date [24]. In the present study, we identified a new natural nisin variant, nisin E, which is produced by _Streptococcus equinus_ MDC1. We have also sequenced the genome of strain MDC1 and identified the 11-gene _nmd_ locus which encodes not only the nisin E precursor peptide but also the protein machinery required for the post-translational modification, processing, and export of the bacteriocin and producer self-immunity. Despite the availability of three other _S. equinus_ genome sequences in GenBank that contain _nmd_-like loci, i.e., those of strains GA-1, PR-5 and SI, this study is the first to demonstrate that nisin E is indeed biologically active, thus expanding the bacteriocin repertoire of the species. In the present work, we have added several species to the screening regime originally reported by [PERSON] et al. [10], mainly those belonging to the anginosus and mitis/oralis streptococcal groups, and the relatively new genus _Macrococcus_[25].