7.2 The eubacterial chromosome
Some of the diverse roles of chromatin components can be illustrated by examining the E. coli chromosome. Like most prokaryotes, E. coli has a single chromosome consisting of a single double-stranded circular DNA molecule. There is no nucleus present, but the E. coli DNA is within a discrete entity in the cytoplasm called the nucleoid. The nucleoid contains a multitude of proteins and is in close proximity to the ribosomes, where translation occurs. In addition to proteins, small, highly positively charged compounds called polyamines are present.
SAQ 31
Based upon what you have learnt about the structure of the DNA double helix, what function do you think polyamines play in packaging DNA?
Answer
The highly positively charged polyamines serve to bind to and counteract the negative charges of the phosphates in the DNA helical backbone, thus aiding compaction by eliminating repulsive forces.
These polyamines include spermine and spermidine, whose structures are shown in Figure 25. They play key roles in assisting with DNA packaging and illustrate a central principle that applies to all DNA packaging systems: in order to compact long DNA strands, the intramolecular repulsive forces must be overcome.
In addition to polyamines, over 20 proteins are found as structural components of E. coli chromatin. They have diverse functions, but we will discuss several that are important in the process of chromosome packaging. The eubacterial chromosome is highly dynamic, with virtually continuous transcription and/or replication. As such, the many proteins found associated with DNA in these organisms are intimately involved in this metabolism, in contrast to the primarily more structural role played by such proteins in eukaryotic cells.
Mitochondria and chloroplasts contain their own genetic material in the form of circular or linear DNA strands that encode various proteins essential for the organelle's function and maintenance. Both these organelles are considered, in evolutionary terms, to have origins in ancient symbiotic relationships with Eubacteria. Support for this view is found in the structure of the chromosomes within these organelles. The DNA in both mitochondria and chloroplasts is packaged with proteins that are related to those found in many Eubacteria.