For example , in Leishmania , protein-coding genes are understood to be organized as large polycistronic units in a head-to-head or tail-to-tail manner ; RNA polymerase II transcribes long polycistronic messages in the absence of defined RNA pol II promoters , and Leishmania has unique features with respect to the regulation of gene expression in response to changes in the environment .
Leishmania has unique features with respect to the regulation of gene expression in response to changes in the environment .