Chapter 30 : Where Art Thou, O Nucleoid?

Abstract:

In a post on May 12, 2008, Elio asked where ribosomes are located in bacterial cells. According to a 2006 paper by Ortiz et al. (1.usa.gov/1NzRno7), “the ribosomes are evenly distributed throughout the cytosol” and “no distinct nucleoid territory was observed.” This prompts one to also ask, “Where art thou, O nucleoid?” Do their observations indeed indicate that, in the small cells of Spiroplasma melliferum ( Figure E ; volume ~0.02 µm3), the ribosomes (number ~1000) and DNA (genome size 1.46 Mbp) co-mingle and that a phase separation between cytoplasm and nucleoid is absent? In larger, fast-growing E. coli cells ( Figure B ; volume 1–3 µm3), the nucleoid is clearly visible in living cells as a low-density compartment, as was already documented by Mason and Powelson in 1956. However, distinct nucleoids are difficult to see in smaller, slow-growing E. coli cells. In the small Caulobacter, nucleoids have even been reported to be absent (R.B. Jensen, 2006) (1.usa.gov/1jQ96ht). The existence of a discrete DNA phase was calculated by Odijk (1998) (1.usa.gov/1RdvvzJ), taking into account the excluded volume interactions between DNA and the soluble proteins as present in small E. coli cells ( Figure A ; volume 0.46 µm3; genome size 4.6 Mbp). To me it seems unlikely that the physical laws that predict the visible phase separation in the larger cells would not hold for the smaller cells, as well.