An analysis of LysM domain function in LytE when fulfilling the D,L-endopeptidase requirement for viability in Bacillus subtilis

The D,L-endopeptidase requirement states that Bacillus subtilis requires either the activity of the LytE or the CwlO enzyme for viability, therefore proving that these two enzymes can complement for each other despite their very different N-terminal domains. Here, we show that another D,L-endopeptidase, LytF, can also fulfill the D,L-endopeptidase requirement for viability, when expressed from the cwlO promoter. Both LytE and LytF contain N-terminally located LysM domains, three and five respectively. However, cells expressing another very similar D,L-endopeptidase CwlS, with four LysM domains were not viable. This led us to investigate whether a LytE protein with any one of its three LysM domains permuted can fulfill the D,L-endopeptidase requirement for viability. We found that the three LysM domains are not functionally equivalent and that the N-terminally located LysM domain plays a greater role for functioning of the LytE enzyme than the subsequent domains. Based on an investigation of orthologous enzymes in 19 B. subtilis species we propose an evolutionary model describing the development of the LytE-, CwlS- and LytF-type D,L-endopeptidases and their LysM domain repeats. In summary, these results show that the LytE enzyme has been optimized to fulfill the D,L-endopeptidase requirement for cell viability of B. subtilis with regard to the number and properties of LysM domains that mediate peptidoglycan-binding.