Chloroplast Genome Organization
All cpDNA molecules are circular and spinach is used as the basis for all comparisons. Very few repeat elements are found other than short sequences of less than 100 bp. The notable exception is a large (10-76 kb) inverted repeat section, which when present, always contains the rRNA genes. (Legumes such as pea do not contain this repeat.) For the majority of species, this repeat region is 22-26 kb in size. Finally,the genetic order of the ribosomal unit is conserved in all species:
16S - tRNAile - tRNAala - 23S - 5S
Recent research has also described two other features of chloroplast DNA. First it was shown to that it can exist in in two orientations This implies that the molecule can undergo an isomerization event. Second is has been shown that spinach, corn, tomato and pea can all exist as multimers (PNAS 86:4156, June 1989).
Because photosysnthesis is the primary function of the chloroplast it is not surprising that the chlroplast genome contains genes which encode for proteins that are involved in that process.
Atrazine resistance is apparantley mediated through the psbA gene sequences of the 32 kd protein which is encoded by cpDNA. DNA sequence analysis revealed the following amino acid changes that are thought to be important.
Evolutionary Changes of cpDNA