Abstract: In this research，tomato（Lycopersicon esculentum）cultivar LA1996 was used as materials. Based on the sequence of MIR390 in mirbase and unigene database of tomato，fragments of premiR390 and potential target LeTAS3 were obtained through PCR using cDNA as template. Transcript levels of LemiR390 and its potential target gene LeTAS3 in tomato fruits were determined by real-time quantitative PCR. LemiR390 guided-cleavage and target site for the putative Trans-Acting SiRNA3（LeTAS3）mRNA was validated using RLM-5′RACE. The results showed that two precursors of miR390 were obtained，with sequences that were consistent with MIR390b and MIR390a and able to form a hairpin structure. Expression of LemiR390 and its target gene LeTAS3 in tomato fruits differed in developmental stage. During the early stage，especially 1–2 weeks after flowering，the transcription levels of LemiR390 and LeTAS3 were high，and LemiR390 negatively regulated the expression of LeTAS3；However，in late stage（3–7 weeks after flowering），their expression levels were low. The mature LemiR390 targeted a sequence of high complementarity in LeTAS3 mRNA and sheared the 10th site of LeTAS4 mRNA from the LemiR390 5? end.

Key words: tomato, LemiR390, LeTAS3, gene expression, miRNA