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. May be required for DNA repair in both organelles. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Not inhibited by aphidicolin. Targeted to chloroplast when translation is initiated at the AUG initiator start, and to mitochondrion when it is initiated at a non-canonical CTG leucine codon located 21-bp upstream of the initiator methionine codon. Expressed in shoot apical meristem. Retarded growth and 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. This sequence can initiate at a non-canonical CTG leucine codon. This non-AUG initiator start codon is located 21-bp upstream of the initiator methionine codon (PubMed:16169894). Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon.