DNA base modification in Chlorella

The nuclear DNA base composition of 26 strains of green algae belonging to the Chlorococcales was analysed by reversed-phase high-performance liquid chromatography (HPLC). 5-Methyldeoxycytidine was the only modified nucleoside detected in addition to the standard nucleosides deoxyadenosine, deoxycytidine, deoxyguanosine and deoxythymidine. The amount of 5-methyldeoxycytidine varied from 1·5 mol% to 12·0 mol% and was correlated with the deoxyguanosine + deoxycytidine (G + C) content of the DNA. No significant influence of deoxycytidine methylation on the thermal stability of the DNA double strand was found. The variation of DNA base composition previously observed within the genus Chlorella and, in particular, within C. sorokiniana, could not be explained by the influence of variable amounts of modified bases on the DNA melting temperature from which the G + C contents were derived.