Abstract
Plastid DNA (ptDNA) is packed with proteins into a highly organized structure called the plastid nucleoid (or plastid nucleus). The pt-nucleoid is thought to be a functional unit of replication and transcription of the plastid genome. Recently, by combining gel filtration and chromatography purification steps, transcriptionally active chromosome (TAC) were isolated from Arabidopsis thaliana and component proteins (pTACs) were identified by mass spectrometry (Pfalz et al. 2006). We focused on one of the pTACs (pTAC5) that has a predicted peptidoglycan binding domain, because we found that the peptidoglycan biosynthetic genes (Mur genes) are related to division of chloroplasts in the moss Physcomitrella patens. In the P. patens genome, only one pTAC5 homologous gene (PppTAC5) was found. Homology search showed that pTAC5 homologs were not found in known algal genomes. Transient expression analysis with a plasmid producing full-length PppTAC5-GFP fusion protein showed that the green fluorescence was observed as many tiny spots in chloroplasts. We are now generating PppTAC5 gene knockout lines in P. patens.
