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• CpMADS4 was decreased during fruit ripening. • CpMADS4 could interact with NAC transcription factor CpNAC3. • Both CpMADS4 and CpNAC3 had transcriptional activation activities. • Both CpMADS4 and CpNAC3 co-regulate ethylene signal genes CpERF9 and CpEIL5. MADS box transcription factors (TFs) play important roles in different biological processes, for example, fruit ripening. However, the roles of MADS box TFs in papaya fruit ripening remain a little know. In this study, a novel MADS-box gene was found, named as CpMADS4 , and its expression level was decreased during fruit ripening. Protein-protein interaction assays proved that CpMADS4 interacted with CpNAC3. Both CpMADS4 and CpNAC3 had transcriptional activation activities. Moreover, CpMADS4 and CpNAC3 could specifically bind to and activate the promoters of ethylene signal genes CpERF9 and CpEIL5 , and the activate activities would be enhanced when both CpMADS4 and CpNAC3 existed at the same time. Our results suggested that CpMADS4 might function in fruit ripening through interacting with CpNAC3 and regulating ethylene signal genes CpERF9 and CpEIL5 , which provided new views about the functions of papaya MADS box TFs in regulating fruit ripening. [ABSTRACT FROM AUTHOR]