Abstract
Nitrogen-fixing Actinobacteria of the genus Frankia can be subdivided into four phylogenetically distinct clades; members of clusters one to three engage in nitrogen-fixing root nodule symbioses with actinorhizal plants. Mur enzymes are responsible for the biosynthesis of the peptidoglycan layer of bacteria. The four Mur ligases, MurC, MurD, MurE, and MurF, catalyse the addition of a short polypeptide to UDP-N-acetylmuramic acid. Frankia strains of cluster-2 and cluster-3 contain two copies of murC, while the strains of cluster-1 and cluster-4 contain only one. Phylogenetically, the protein encoded by the murC gene shared only by cluster-2 and cluster-3, termed MurC1, groups with MurC proteins of other Actinobacteria. The protein encoded by the murC gene found in all Frankia strains, MurC2, shows a higher similarity to the MurC proteins of plants than of Actinobacteria. MurC2 could have been either acquired via horizontal gene transfer or via gene duplication and convergent evolution, while murC1 was subsequently lost in the cluster-1 and cluster-4 strains. In the nodules induced by the cluster-2 strains, the expression levels of murC2 were significantly higher than those of murC1. Thus, there is clear sequence divergence between both types of Frankia MurC, and Frankia murC1 is in the process of being replaced by murC2, indicating selection in favour of murC2. Nevertheless, protein modelling showed no major structural differences between the MurCs from any phylogenetic group examined.
Highlights
Bacteria are exposed to various environmental stresses against which the cell wall has to provide protection, while maintaining the cell shape
This study aimed to answer the questions if one of the two Frankia murC copies originated from gene duplication or from horizontal gene transfer (HGT), and whether there is evidence for subfunctionalization
MurC2 in the genomes of the Frankia strains from cluster-1
Summary
Bacteria are exposed to various environmental stresses against which the cell wall has to provide protection, while maintaining the cell shape. Intracellular symbiotic bacteria need to survive both inside and outside the host, which can create the need for cell wall modifications. The cell walls of Gram-positive bacteria are characterized by a ca. As a much thinner layer in the cell wall, PG can be found in Gram-negative bacteria (3–6 nm), cyanobacteria (10 nm), and moss chloroplasts (5 nm) [1,2,3,4]. PG is synthesized by the activity of Mur ligases, which add a number of amino acids to UDP-N-acetylmuramic acid to connect different strands. In a series of ATP-dependent reactions, L-alanine (MurC), D-glutamate (MurD), L-lysine (MurE), and D-alanyl-D-alanine (MurF) are added sequentially. Several reports on the activity of Mur ligases have been published for non-pathogenic bacteria. PG layers have not been identified for the chloroplasts of Spermatophyta, it is known that Arabidopsis thaliana
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