In addition to polymerase activity, this DNA polymerase exhibits 5'-3' exonuclease activity (By similarity). Required for DNA replication and accumulation in plastids and mitochondria. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Not inhibited by aphidicolin. A number of isoforms are produced. According EST sequences. Expressed in shoot apical meristem. No visible phenotype under normal growth conditions, but mutant plants have reduced levels of DNA in both mitochondria and plastids. Double homozygous mutants polIa and polIb are sterile. Belongs to the DNA polymerase type-A family.