Properties of the satellite DNA associated with the chloroplasts of Euglena gracilis

Abstract

Chloroplasts of the algal flagellate Euglena gracilis were isolated by sucrose gradient sedimentation of crude lysates and were shown to be enriched with a satellite DNA component. The satellite and principal DNA components were separated in a cesium chloride density-gradient and compared with respect to buoyant density, thermal denaturation, base composition and molecular weight distribution. The satellite DNA (ρ = 1·685 g/cc) had a lower melting temperature than the principal DNA (ρ = 1·707 g/cc). Consistent with this was a high adenine—thymine content; 76% as compared to 47% in the principal DNA. However, the principal DNA contained 2·3 mole per cent of the rare base 5-methylcytosine, whereas none could be detected in the satellite DNA. The molecular weight range of the principal DNA was 20 to 40 million. More than half of the satellite DNA appeared in a comparable molecular weight range; but the remainder was less heterogeneous, with lower molecular weight of 2·6 million. The component of small molecular weight is probably not a breakage product of the larger one.