Heterogeneity and complexity of Chlorella chloroplastic DNA.

Abstract

The heterogeneity and complexity of Chlorella chloroplastic DNA has been investigated by means of thermal denaturation and renaturation kinetics.The chloroplastic DNA presents a Tm of 82.2°C, a maximal hyperchromicity of 40.2% and a dispersion coefficient Δ2/3 of 6°C. The absorbance melting profiles display irregularities indicating an intramolecular heterogeneity of the DNA. The first derivated curves of the melting profiles show the presence in the chloroplastic DNA of two main components of Tm 81 °C and 84.7 °C (which represent together about 80% of the total DNA) and possibly three other minor ones.Denatured and sheared DNAs renature rapidly as a fast and a slow‐renaturing class with a kinetic complexity of 3 · 107 daltons and 2.3 · 108 daltons respectively (assuming a value of 2.5 · 109 daltons for Escherichia coli DNA). If correction is applied to account for the low d(G + C) content of the chloroplastic DNA, the kinetic complexity of the slow‐renaturing component is found to be in the range of 1.2‐1.3 · 108 daltons.Our results confirm and extend the fact that chloroplastic DNAs are similar in terms of kinetic complexity and heterogeneity. This is in good agreement with a possible common origin for all the chloroplastic DNAs.From the discrepancy between analytical and kinetic complexities of Chlorella chloroplastic DNA, one can assume that this DNA is extensively reiterated and that the unique nucleotide sequence is present in about 20 copies in the chloroplastic genome.