The Organization of the Trypanosoma brucei Nucleus

Taken from Professor Chorba's Biology Website at
Mercer County Community College

The interphase nucleus in T. brucei is approximately 3 µm in diameter and has a single 0.5-1 µm nucleolus. Electron-dense and translucent areas can be seen under microscopic observation. These may be involved in regulating gene expression because there is a greater amount of electron-dense regions in the bloodstream form as opposed to the procyclic form (Ersfeld, et al., 1999).

There is an evidence of regulatory communication between kinetoplasts and the nucleus because the S phases of mitosis for each are linked periodically. For each, segregation of the replicated genomes involves microtubule formation from basal bodies that are duplicated. Nuclear mitosis occurs within an intact nuclear envelope, which is unusual compared to other eukaryotes. In addition, there are no known centromeric regions, so the microtubules terminate at kinetochore-like structures. It has been estimated that there are 7-8 of these kinetochore-like structures in a metaphase cell (Ersfeld, et al., 1999).

There is approximately 3.5x10^7 bp per haploid genome of total nuclear DNA in trypanosomes. Three types of chromosomes exist in the trypanosome nucleus (Ersfeld, et al., 1999):

Minichromosomes (MCs): there are approximately 100 MCs in the nucleus and they vary from 50-150 kb in size. 90% of MCs contain a 177 bp repeat of unknown function. The region between one of the telomeres and the 177 bp repeat contains 74 bp GC-rich sequences separated by a 155 bp AT-rich sequences. MCs lack protein-coding genes except many may contain one or two inactive copies of the variant surface glycoprotein (VSG) genes near the telomeres.

Intermediate chromosomes (ICs): there are 1 to 5 ICs in the nucleus that range from 200-900 kb and lack the 177 bp repeat. ICs contain the silent VSG expression sites.

Megabase chromosomes (MBCs): there are 11 pairs of MBCs in the nucleus that range from 1 to greater than 6 Mbp. The homologous chromosomes greatly differ in size. MBCs carry the rest of the active genes. Many of these genes are arranged in tandem repeats that contribute to the immense size polymorphisms.

Sources:

Ersfeld, K., Melville, S.E, & Gull, K. (1999). Nuclear and genome organization of Trypanosoma brucei. Parasitology Today 15: 58-64.

Created by Margaret Maddux as part of a senior seminar at Earlham College

Last updated: April 10, 2003