A gene from the VSG expression site ofTrypanosoma bruceiencodes a protein with both leucine-rich repeats and a putative zinc finger

Abstract

The transcription unit of the gene for the variant specific glycoprotein (VSG) AnTat 1.3A of Trypanosoma brucei contains several associated genes (ESAGs, for Expression Site-Associated Genes), 7 of which have already been described. We report here the characterization of a further ESAG, which we term ESAG 8, present 1 kb downstream from the putative adenylate cyclase gene ESAG 4. ESAG 8 encodes a 70 kd protein whose sequence indicates that it is probably not exposed at the cell surface. With the exception of the N-terminal domain which contains a presumptive DNA-binding zinc finger, the ESAG 8 protein consists exclusively of leucine-rich repeats of 23 amino acids, typical of protein-interacting domains such as the RAS-interacting region of the yeast adenylate cyclase. ESAG 8 transcripts are only found in bloodstream forms, and their level is particularly low, suggesting a high rate of degradation. The ESAG 8 protein may be involved in stage-specific regulatory processes, such as gene expression control or adenylate cyclase activation.

A gene from the variant surface glycoprotein expression site encodes one of several transmembrane adenylate cyclases located on the flagellum of Trypanosoma brucei.

“…This observation is in accordance with the fact that the trypanosome cyclase genes differ from that of S. cerevisiae except for the region encoding the catalytic domain of the enzyme (2,21). In particular, the trypanosome cyclases are devoid of the leucine-rich domain known to be involved in activation by Ras in yeast cells (23). It is interesting to note that a gene encoding such a leucinerich domain (ESAG 8) is located close to ESAG 4 in the VSG gene expression site (23,27).…”

“…As discussed previously (23,27), the trypanosome cyclase genes appear to lack the region which, in S. cerevisiae, is responsible for interaction with Ras. To determine whether the ESAG 4 and GRESAG 4.1 activities can be regulated in yeast cells, the complemented yeast cells were subjected to nitrogen starvation and heat shock.…”

“…The VSG is no longer synthesized in the procyclic (insect) form of the parasite (for a recent review, see reference 7). The VSG gene is transcribed in a telomeric expression site, together with a battery of additional genes (ESAGs, for expression site-associated genes) (1,8,14,16,21,23). Among these, ESAG 4 shares with three related sequences (GRESAGs, for genes related to ESAGs) a 3'-terminal sequence likely to encode the catalytic domain of either adenylate or guanylate cyclase, since this region is homologous to the C-terminal catalytic domain of both the adenylate cyclase of Saccharomyces cerevisiae and the membrane form of rat brain guanylate cyclase (2).…”

Accurate polyadenylation of procyclin mRNAs in Trypanosoma brucei is determined by pyrimidine-rich elements in the intergenic regions.

“…While all of the procyclin expression sites appear to be transcribed in procyclic forms of the parasite (12, 13, 19), only one VSG expression site at a time is active in bloodstream-form trypanosomes (5). The active VSG transcription unit is extremely large and encompasses at least eight expression site-associated genes (ESAGs) between the promoter and the VSG gene (23,25).We have recently begun to define regulatory elements associated with procyclin genes and have shown that correct polyadenylation of transcripts can occur in the absence of the splice acceptor site from the downstream gene (9). In this study we have now localized a pyrimidine-rich element in the intergenic region between tandemly linked procyclin genes and have shown that it is required for accurate polyadenylation of mRNAs.…”

“…While all of the procyclin expression sites appear to be transcribed in procyclic forms of the parasite (12, 13, 19), only one VSG expression site at a time is active in bloodstream-form trypanosomes (5). The active VSG transcription unit is extremely large and encompasses at least eight expression site-associated genes (ESAGs) between the promoter and the VSG gene (23,25).…”