The autolytic peptidoglycan hydrolases of Streptococcus faecium

Identification of a gene (arpU) controlling muramidase-2 export in Enterococcus hirae

Muramidase-2 of Enterococcus hirae is a 74-kDa peptidoglycan hydrolase that plays a role in cell wall growth and division. To study its regulation, we isolated a mutant defective in muramidase-2 release under certain growth conditions. This mutant had cell walls which apparently lacked 74-kDa muramidase-2 but which accumulated two proteolytic fragments of 32 and 43 kDa, which exhibited muramidase-2 activity in the membrane fraction. By complementation cloning, we identified a 2.6-kb fragment of the E. hirae chromosome containing a gene cluster coding for proteins of 58 to 137 amino acids. One of these genes (arpU), which encoded a 15.9-kDa protein, was shown to complement the defect of the A9 mutant in trans. We propose that this gene may be involved in the regulation of muramidase-2 export.

Is there a correlation between membrane phospholipid metabolism and cell division?

The specificity of lipid/protein interactions in bacterial membranes is based on the diversity and variability of whole phospholipid molecules and of their polar and non-polar moieties. Of particular interest is the synthesis and turnover of anionic phospholipids facilitating variations in the phosphatidylglycerol (PG)/diphosphatidylglycerol (DPG) ratio in correlation with cell growth and division. Accumulation of DPG increases the anionic character of the cell membrane which is known to hinder the septation process. Consequently, the decrease in PG/DPG observed in stationary and in penicillin- or UV-inhibited non-dividing bacteria can be explained in this manner. On the other hand, the presence of lysophosphatidylethanolamine observed in the chain-forming mutant envC PM61 of Escherichia coli, and resulting from an anomaly in the deacylation-reacylation cycle, appears to be an indirect consequence rather than the cause of septation deficiency. Variations in the ratio of saturated/unsaturated fatty acids in phospholipids are also correlated with the physiological state of the cell. Hence, the levels of fluid and non-fluid phospholipids, their polymorphism and interaction with membrane proteins appear to be implicated in cell wall modelling. Future work will show how all these correlations function on the genetic and molecular levels.