Entire maize chloroplast genome is stably maintained in a yeast artificial chromosome

The 139 kb circular maize chloroplast genome was cloned in a linear form in a single yeast artificial chromosome (YAC), and several such clones were then analyzed to study the stability of large DNA sequences in YACs. The YAC clones were isolated from a partial maize DNA YAC library. The library was constructed by Bal-31 nuclease digestion of high-molecular-weight total maize genomic DNA followed by blunt-end ligation into a modified YAC vector, pYAC11. All chloroplast genome-containing YACs (cpYACs) were characterized by comparing their restriction enzyme digestion patterns with that of purified maize chloroplast DNA (MCD). Comparison of the restriction patterns of four cpYACs, MY503, MY504, MY518 and MY526, with that of MCD did not show any change in the size of the restriction fragments except at the chloroplast DNA-vector junction fragments. These altered size fragments were confirmed as junction fragments by hybridization of the pYAC11 border fragments. The end-specific T3 and T7 transcripts synthesized from MY503 also identified the same junction fragments as did the pYAC11 border fragments. Most cpYACs were completely stable through the analyzed one hundred generations.