Results 
Discovery and Characterization of SalK/SalR The newly decoded genomic sequence of S. suis 05ZYH33 makes it possible to systematically investigate the genetic basis of streptococcal pathogenicity.
We focused on the putative 89K PAI to perform further molecular analysis.
On the negative strand of 89K, peptides encoded by 05SSU0944 and 05SSU0943 exhibit 27% and 41% amino acid sequence identity with the SalK and the SalR proteins of Streptococcus salivarius, respectively, forming a TCSTS belonging to the Nar/LuxR family.
05SSU0944 and 05SSU0943 were accordingly renamed salK and salR (Fig. 1).
Notably, the average GC content of salKR, 29.9%, is much less than that of both 89K and the whole genome of 05ZYH33 (36.8% and 41.1%, respectively) (Fig. 1B).
The results of bioinformatics analysis indicated that the response regulator, SalR, contains 199 amino acids (aa) with the three N-terminal invariant aa residues (D30, D54 and K107) of the receiver domain of response regulators from Gram-positive bacteria [25].
In the C-terminal domain, it contains two residues (E142, G152) that are absolutely conserved in the winged helix-turn-helix DNA-binding domains of response regulators belonging to the LuxR family [26].
salK encodes a 396 aa long protein, with five N-terminal transmembrane domains and a transmitter domain signature motif in the C-terminal - typical of sensor histidine protein kinases of TCSTSs [25].
As some TCSTS gene pairs are adjacent to the genes that they control, the genetic structure of the salKR locus and its flanking genes was defined (Fig. 2A).
The salK coding region overlaps that of salR by 20 base pairs (bp), and no putative promoter or transcriptional terminator could be identified within either gene.
A putative promoter located approximately 50 bp upstream from the TTG initiation codon of salK, was predicted using the online server of Neutral Network Promoter Prediction.
Further sequence analysis revealed a palindrome of 55 bp with the potential to form a stem-loop structure situated in the intergenic region between salR and 05SSU0942, which may function as a transcriptional terminator.
05SSU0945, situated upstream (729 bp long intergenic region) of the salKR locus, encodes a 94 aa polypeptide for which there were no significant database matches.
Downstream of salKR, the 05SSU0942, 05SSU0941, 05SSU0940, 05SSU0939, 05SSU0938 genes were predicted to encode peptides with significant identities to the orthologues of Streptococcus agalactiae.
The function of these downstream gene products are unknown except for 05SSU0941, which encodes a predicted DNA primase that might be involed in DNA replication.
The genetic organization of this locus indicates that salK might be cotranscribed with salR initiating from the PsalK promoter.
