The v-rel oncoprotein encoded by reticuloendotheliosis virus T strain (Rev-T) is a member of the rel/NF-kappaB family of transcription factors capable of transformation of primary chicken spleen and bone marrow cells. Rapid transformation of avian haematopoietic cells by v-rel occurs through a process of deregulation of multiple protein-encoding genes through its direct effect on their promoters. More recently, upregulation of oncogenic miR-155 and its precursor pre-miR-155 were demonstrated in Rev-T-infected chicken embryo fibroblast cultures as well as Rev-T-induced B-cell lymphomas. Through electrophoresis mobility shift assay and reporter analysis on gga-miR-155 promoter, we show that the v-rel-induced miR-155 overexpression occurs by the direct binding to one of the putative NF-kappaB binding sites. Using v-rel-induced transformation model on chicken embryonic splenocyte cultures, we could demonstrate dynamic increase in miR-155 levels during the transformation. Transcriptome profiles of lymphoid cells transformed by v-rel showed upregulation of miR-155 accompanied by downregulation of a number of putative miR-155 targets such as Pu.1 and CEBPbeta. We also show that v-rel can rescue the suppression of miR-155 expression observed in Marek's disease virus-transformed cell lines, where its functional viral homolog MDV-miR-M4 is overexpressed. Demonstration of gene expression changes affecting major molecular pathways including organismal injury and cancer in avian macrophages transfected with synthetic mature miR-155 underline its potential direct role in transformation. Our study suggests that v-rel-induced transformation involves complex set of events mediated by the direct activation of NF-kappaB targets together with the inhibitory effects on miRNA targets.