TALE activation of endogenous genes in Chlamydomonas reinhardtii

a r t i c l e i n f o Background: Transcription activator-like effectors (TALEs) are effector proteins secreted by certain plant pathogenic bacteria when infecting their hosts. Upon translocation, TALEs bind via a well-deciphered recognition code to specific sequences in the promoter region of targeted host genes, thereby activating expression of those targeted genes. Gene activation induced by artificially designed TALEs (dTALEs) has been studied in multiple organisms, but is yet to be demonstrated in green algae, such as Chlamydomonas reinhardtii, a well-known model organism for fundamental biological studies, as well as for biofuel production. Results: We chose two endogenous Chlamydomonas genes, ARS1 and ARS2, as the targets of dTALE induced activation. Both genes encode a periplasmic arylsulfatase (ARS), and are located in an apparent tail-to-tail inverted duplication on chromosome 16. dTALEs independently targeting ARS1 and ARS2 promoters were generated and successfully expressed in Chlamydomonas. Both target genes exhibited noticeably increased expression induced by their respective dTALEs at the transcript level as well as the protein level, which was confirmed at the protein activity level by ARS colorimetric assays. The level of induced target gene activation was closely correlated with the dTALE transcript abundance. Conclusions: Our work demonstrates robust gene-specific activation induced by artificially designed TALEs in the green alga Chlamydomonas. The frequency and efficiency of the induced expression demonstrate dTALEs as powerful tools for targeted gene activation in Chlamydomonas. The pattern of activation shown here provides insights into the mechanism of TALE induced expression, and also confirms the activity of the activation domain of naturally occurring TALEs in another organism. Our success with dTALE-induced activation in Chlamydomonas may open a new avenue to fast, high throughput gene manipulation in other related organisms, such as green algae and crop plants. Transcription activator-like effectors (TALEs) are disease-related proteins secreted by certain plant pathogenic bacteria, e.g., those of the genus Xanthomonas [2]. Through their type III secretion system, Xanthomonas cells deliver TALEs to host plant cells during the infection [5,6,61]. These proteins function as transcription factors by binding to the promoter sequences of specific host target genes. This results in activation of host resistance genes or susceptibility genes and, therefore, triggers either resistance responses or disease susceptibility in the host [4,56,57]. All known TALEs typically contain three major domains: the N-terminal secretion signal, the C-terminal nuclear localization domain and trans-activation domain, and the central region of 1.5– 33.5 repeats of …