The Peptidoglycan Biosynthesis Genes MurA and MraY are Related to Chloroplast Division in the Moss Physcomitrella patens

Abstract

In the moss Physcomitrella patens, 10 Mur genes involved in peptidoglycan biosynthesis were found, and the MurE and Pbp genes are related to plastid division. Although the MraY and MurG genes were missing in our previous expressed sequence tag screening, they were discovered in the P. patens genome in this study, indicating that P. patens has a full set of genes capable of synthesizing peptidoglycan. In addition, a second MurA gene (PpMurA2) was found. Whereas Northern analyses indicated that PpMurA1, PpMurG and PpMraY were expressed, transcripts of PpMurA2 were detected only when RT-PCR was employed. Whereas GFP fusion proteins with either PpMurA1 or PpMraY were detected in chloroplasts, the PpMurA2 fusion proteins were located in the cytoplasm. Protonema cells in the wild-type plants had an average of 46 chloroplasts. PpMurA1 gene-disrupted lines had <10 chloroplasts, whereas approximately 30 chloroplasts existed in the PpMurA2 knockout lines. The PpMurA1/A2 double-knockout lines had only a few macrochloroplasts, suggesting a redundant function for these two genes. Disruption of the PpMraY gene in P. patens resulted in the appearance of macrochloroplasts. Anabaena MraY fused to the N-terminal region of PpMraY and A. thaliana MraY could complement the macrochloroplast phenotype in the PpMraY knockout line. Electron microscopic observations showed no obvious differences in the shape or stacking of thylakoid membranes between all knockout transformants and wild-type plants, suggesting that these Mur genes are related only to plastid division in moss.

Mechanism of Plastid Division: From a Bacterium to an Organelle

“…Disruption of the nucleus-encoded mur or mra genes impairs chloroplast division in the moss P. patens (Machida et al, 2006;Homi et al, 2009). The chloroplast genomes of certain green algal lineages still retain ftsI and ftsW genes.…”

Moss Chloroplasts Are Surrounded by a Peptidoglycan Wall Containing D-Amino Acids

“…However, we and others have found that antibiotics that interfere with peptidoglycan biosynthesis can inhibit plastid division, generating giant chloroplasts, in the moss Physcomitrella patens (Kasten and Reski, 1997;Katayama et al, 2003). Due to progress in the P. patens genome-sequencing project, we were able to identify 10 homologs of Mur genes: MurA to G, MraY, penicillin binding protein (PBP), and D-Ala:D-Ala ligase (DDL), which are involved in biosynthesizing peptidoglycan (Machida et al, 2006;Homi et al, 2009). Disruption of the P. patens MurA, MurE, MraY, or Pbp resulted in the appearance of a smaller number of macrochloroplasts in each protonema cell, in contrast to wild-type plant cells, which have ;50 chloroplasts (Machida et al, 2006;Homi et al, 2009).…”

“…Due to progress in the P. patens genome-sequencing project, we were able to identify 10 homologs of Mur genes: MurA to G, MraY, penicillin binding protein (PBP), and D-Ala:D-Ala ligase (DDL), which are involved in biosynthesizing peptidoglycan (Machida et al, 2006;Homi et al, 2009). Disruption of the P. patens MurA, MurE, MraY, or Pbp resulted in the appearance of a smaller number of macrochloroplasts in each protonema cell, in contrast to wild-type plant cells, which have ;50 chloroplasts (Machida et al, 2006;Homi et al, 2009). MurE mediates the formation of UDP-Nacetylmuramic acid (MurNAc)-tripeptide in bacteria ( Figure 1A).…”

Evolution of the chloroplast division machinery

“…With the sequenced genome of moss (Rensing et al, 2008), genes for peptidoglycan biosynthesis pathway were discovered (Takano and Takechi, 2010). Knockout of PpPbp, PpMurE, PpMurA and PpMraY genes all showed a phenotype of enlarged chloroplasts (Machida et al, 2006;Homi et al, 2009), suggesting a role of chloroplast division of these genes. It needs to be further investigated whether the chloroplast in moss really has a peptidoglycan wall and whether this wall is related to chloroplast division if it does exist.…”