Size-fractionation of the small chromosomes of Trypanozoon and Nannomonas trypanosomes by pulsed field gradient gel electrophoresis.

We have compared the molecular karyotypes of trypanosomes from different subgroups within subgenus Trypanozoon by pulsed field gradient (PFG) gel electrophoresis. Although the overall karyotype was similar, there was much variation in the size of chromosomes between different stocks. Two of three stocks of Trypanosoma (Trypanozoon) brucei gambiense had remarkably small mini-chromosomes: 25-50 kilobase pairs compared to 50-150 kilobase pairs for the mini-chromosomes of other Trypanozoon stocks. The relative amount of DNA in the mini-chromosomal fraction of different stocks correlated well with the amount of 177 base pair satellite DNA monomer per microgram nuclear DNA. Hybridisation of Southern blots of pulsed field gradient gels with a number of gene probes showed that the loci for tubulin and phosphoglycerate kinase in Trypanozoon probably lie on the same chromosome, together with some variant surface glycoprotein genes; the genes for triose phosphate isomerase and glyceraldehyde phosphate dehydrogenase are separately located both with respect to each other and the above housekeeping genes. Therefore, there are at minimum three pairs of chromosomes carrying housekeeping genes in Trypanozoon. In some stocks the chromosomes carrying the tubulin and phosphoglycerate kinase genes are split into two bands, suggesting that homologous chromosomes may differ substantially in size in trypanosomes. One Trypanosoma (Nannomonas) congolense stock examined had a similar pattern of chromosome distribution to that of Trypanozoon, but with very small mini-chromosomes (25-50 kilobase pairs.)