GROWTH-PHASE-DEPENDENT SYNTHESIS OF HISTONES IN THE ARCHAEON METHANOTHERMUS-FERVIDUS

Histone preparations from Methanothermus fervidus (HMf) contain two sm all polypeptides, HMfA and HMfB, which in solution are dimers and comp act DNA to form nucleosome-like structures, These archaeal nucleosome- like structures constrain positive DNA supercoils, in contrast to the negatively supercoiled DNA in eukaryal nucleosomes. HMfA has been foun d to make up as much as 80% of HMf preparations synthesized by M. ferv idus cells during the exponential growth phase of batch cultures but t o decrease to approximate to 50% as cultures enter the stationary phas e. By using a nondenaturing polyacrylamide gel system at pH 6.1, we ha ve demonstrated that HMf preparations contain HMfA homodimers, HMfB ho modimers, and HMfA-HMfB heterodimers and that heating a mixture of rec ombinant HMfA and HMfB homodimers at 95 degrees C for 5 min generates HMfA-HMfB heterodimers. Circular dichroism spectroscopy indicates that HMfA and HMfB have very similar secondary structures, but based on ag arose gel electrophoretic mobility shifts, DNA topology assays, and el ectron microscopy, they have different DNA binding properties. HMfA bi nding to DNA could be detected at lower protein/DNA ratios than HMfB, but HMfB binding resulted in more extensive DNA compaction. The increa sed HMfB synthesized in cells approaching the stationary phase and the highly compacted state of HMfB-bound DNA are consistent with preparat ions for the impending period of limited genome activity.