aureus 8325-4 cells. To observe whether there was any impact of LytM deletion on S. aureus autolysis, the lytM mutation was transferred to the S. aureus strain 8325-4 and the lyt− transposon mutant of strain 8325-4. There was no appreciable difference in the autolysis of the lytM mutant cells of strain 8325-4 relative to wild-type 8325-4 (Fig. 4). Additionally, no autolysis was observed in the case of the lyt− and lyt−:lytM double mutant during the course of the experiment (5 h) when autolysis was EX 527 cell line measured periodically (Fig. 4). The turbidity of the lyt− and lyt−:lytM cell suspension remained unchanged even after 24 h (data not shown). In zymographic investigations, several lytic-activity bands were
seen in samples Selleckchem Fluorouracil from the wild-type S. aureus strain 8325-4 (Fig. 5, lane 1). The pattern of autolytic bands was almost identical in samples from the lytM mutant of S. aureus strain 8325-4 (Fig. 5, lane 3). In these experiments,
the S. aureus lyt−:lytM double mutant was expected to be autolysin free based on the previous report that suggested the LytM protein to be responsible for the residual autolytic activity in the lyt−S. aureus (Ramadurai & Jayaswal, 1997). Surprisingly, in the zymographic investigations, the pronounced 36 kDa lytic activity band in lyt−S. aureus (Fig. 5, lane 2), postulated to be due to LytM, was present in the lyt−:lytM double mutant (Fig. 5, lane 4). This observation suggests that LytM is not responsible for the residual activity of the lyt− strain of S. aureus. To address the presence of the 36 kDa lytic activity band in the lyt−:lytM double mutant, the lytM gene was cloned in vector pRSETA and overexpressed in
E. coli. The protein band that appeared to be induced after the addition of IPTG was a 36 kDa protein (Fig. 6a, arrow comparing lanes 2 and 3). The size expected for the old full-length His-tagged LytM was 40 kDa. The protein that was repeatedly purified following metal chromatography was also 40 kDa in size (Fig. 6a, lane 1). It has been reported that the LytM signal peptide undergoes cleavage even in E. coli cells (Ramadurai & Jayaswal, 1997; Odintsov et al., 2004). This leads to the loss of the signal peptide and the approximately 4 kDa His-tag present on the N-terminus of the recombinant His-tagged LytM. It is speculated that the majority of the overexpressed LytM undergoes cleavage of the signal peptide and only a small fraction of LytM remains intact with the His-tag, which could be purified. In zymographic experiments, Ramadurai & Jayaswal (1997) reported three autolysin bands of 36, 22 and 19 kDa in extracts of E. coli cells that overproduced LytM and proposed that the lower lytic-activity bands were LytM-degraded products. However, in our zymographic experiments, no autolytic band was visualized even after prolonged incubation of the zymographic gel in the lane corresponding to purified His-tagged LytM (Fig. 6b, lane 4).