A short bifunctional element operates to positively or negatively regulate ESAG9 expression in different developmental forms of Trypanosoma brucei.

In their mammalian host trypanosomes generate ‘stumpy’ forms from proliferative ‘slender’ forms as an adaptation for transmission to their tsetse fly vector. This transition is characterised by the repression of many genes while quiescent stumpy forms accumulate during each wave of parasitaemia. However, a subset of genes are upregulated either as an adaptation for transmission or to sustain infection chronicity. Among this group are ESAG9 proteins, whose genes were originally identified as a component of some telomeric variant surface glycoprotein gene expression sites, although many members of this diverse family are also transcribed elsewhere in the genome. ESAG9 genes are among the most highly regulated genes in transmissible stumpy forms, encoding a group of secreted proteins of cryptic function. To understand their developmental silencing in slender forms and activation in stumpy forms, the post-transcriptional control signals for a well conserved ESAG9 gene have been mapped. This identified a precise RNA sequence element of 34 nucleotides that contributes to gene expression silencing in slender forms but also acts positively, activating gene expression in stumpy forms. We predict that this bifunctional RNA sequence element is targeted by competing negative and positive regulatory factors in distinct developmental forms of the parasite. Analysis of the 3′UTR regulatory regions flanking the highly diverse ESAG9 family reveals that the linear regulatory sequence is not highly conserved, suggesting that RNA structure is important for interactions with regulatory proteins.