Inverted repeats in chloroplast DNA from higher plants

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Abstract
The circular chloroplast DNA from spinach [Spinacia oleracea], lettuce [Lactuca sativa] and corn [Zea mays] plants were examined by EM and shown to contain a large sequence repeated 1 time in reverse polarity. The inverted sequence in spinach and lettuce chloroplast DNA is 24,400 base pairs long. The inverted sequence in the corn chloroplast DNA is 22,500 base pairs long. Denaturation mapping studies showed that the structure of the inverted sequence is highly conserved in these 3 plants. Pea [Pisum sativum] chloroplast DNA does not contain an inverted repeat. All of the circular dimers of pea chloroplast DNA are in a head-to-tail conformation. Circular dimers of spinach and lettuce also had head-to-tail conformation. However, approximately 70-80% of the circular dimers in preparations of lettuce and spinach chloroplast DNA were in a head-to-head conformation. The head-to-head circular dimers probably are formed by a recombination event between 2 circular monomers in the inverted sequence.