MicroRNAs regulate gene expression as part of the RNA-induced silencing complex, for which the sequence identity of the miRNA provides the specificity to the target messenger RNA and the result is target repression. The mode of repression can be through target cleavage, RNA destabilization and/or decreased translational efficiency. Here, we provide a comprehensive global analysis of the evolutionarily distant unicellular green alga Chlamydomonas reinhardtii to quantify the effects of miRNA on protein synthesis and RNA abundance. We show that, similar to metazoan steady-state systems, endogenous miRNAs in Chlamydomonas can regulate gene-expression both by destabilization of the mRNA and by translational repression. However, unlike metazoan miRNA where target site utilization localizes mainly to 3'UTRs, in Chlamydomonas utilized target sites lie predominantly within coding regions. These results demonstrate the evolutionarily conserved mode of action for miRNAs, but details of the mechanism diverge between plant and metazoan kingdoms.This work was supported by a Balzan Prize award and the European Research Council Advanced Investigator Grant ERC-2013-AdG 340642 TRIBE. B.Y.W.C. was supported by an EMBO long-term postdoctoral fellowship and a Sir Henry Wellcome Fellowship [096082]. D.C.B. is the Royal Society Edward Penley Abraham Research Professor
