Cloning and expression analysis of genes encoding lytic endopeptidases L1 and L5 from Lysobacter sp. strain XL1

Lytic enzymes are the group of hydrolases that break down structural polymers of the cell walls of various microorganisms. In this work, we determined the nucleotide sequences of the Lysobacter sp. strain XL1 alpA and alpB genes, which code for, respectively, secreted lytic endopeptidases L1 (AlpA) and L5 (AlpB). In silico analysis of their amino acid sequences showed these endopeptidases to be homologous proteins synthesized as precursors similar in structural organization: the mature enzyme sequence is preceded by an N-terminal signal peptide and a pro region. On the basis of phylogenetic analysis, endopeptidases AlpA and AlpB were assigned to the S1E family [clan PA(S)] of serine peptidases. Expression of the alpA and alpB open reading frames (ORFs) in Escherichia coli confirmed that they code for functionally active lytic enzymes. Each ORF was predicted to have the Shine-Dalgarno sequence located at a canonical distance from the start codon and a potential Rho-independent transcription terminator immediately after the stop codon. The alpA and alpB mRNAs were experimentally found to be monocistronic; transcription start points were determined for both mRNAs. The synthesis of the alpA and alpB mRNAs was shown to occur predominantly in the late logarithmic growth phase. The amount of alpA mRNA in cells of Lysobacter sp. strain XL1 was much higher, which correlates with greater production of endopeptidase L1 than of L5.