Archaeal chromosome biology

Abstract

Knowledge of the chromosome biology of archaeal species has grown considerably in the last 15 years, since the publication of the first full archaeal genome sequences. A number of model organisms have been studied, revealing a striking variety of mechanisms and modes of genome duplication and segregation. While clear sequence relationships between archaeal and eukaryotic replication proteins are well known, some archaea also seem to possess organizational parameters for replication and segregation that reveal further striking parallels to eukaryotes.

Figure 1

Schematic representation of marker frequency analysis of a hypothetical organism with two replication origins, oriC1 and oriC2. In the scenario depicted, oriC2 fires later in the cell cycle than oriC1. The four representative chromosomes shown are in various stages of replication, representing examples from an asynchronous culture. The copy numbers of the individual loci from all four molecules are tabulated and presented as a graph of copy number versus genome position. Two peaks can be observed, corresponding to oriC1 and oriC2, the lower amplitude of the oriC2 peak, due to the later replication and thus lower copy number (ΔCN) than oriC1, is indicated. Note that this is a simplified example in which the chromosomes are all replicating. The presence of non-replicating chromosomes will damped down the ratio from the optimal 2:1 origin:terminus shown in this cartoon while retaining the same overall profile.

Figure 2

Organization of the E. coli and S. solfatricus chromosomes (panels A and B respectively). Replication origins are shown by open circles, the principle site-specific termination sites (TerA, B, C and D) are indicated by double wedge shapes, the dif site locus by a triangle and the fork fusion zones (ffz) by ovals. In E. coli, the independent left and right replichores are shown by bold arrows.