Unique nucleoid structure during cell division of Thermococcus kodakaraensis KOD1

The nucleoid structure and the partition in the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 were observed by a combination of phase-con trast microscopy and fluorescence microscopy. The nucleoids occurred as rou nded fluorescent foci centrally located in the cells and as differences in fluorescence intensity between exponential and stationary phases. The cellu lar space occupied by the nucleoid in the stationary phase was larger than that in the exponential phase. Various shapes of nucleoid in the exponentia l-phase cells were observed, indicating that nucleoid separation was proces sed under cell cycle control. The number of cells which showed distinctive division stages was counted and the proportions of dividing cells were dete rmined. About half of the observed cells were in the replication stage. Mor e than 40% of the counted cells possessed a fully replicated but not separa ted form of nucleoid. Only 8% of the total cells clearly showed visible con striction. These results suggested that the post-replication period before cell division was relatively as long as the eucaryal gap period (G2); howev er, the period of visible cell constriction was almost the same as that of the bacteria.