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• We identified mcr-1 gene in fifteen clinical isolates of Enterobacteriaceae. • The mcr-1 -harboring plasmids belonged to different incompatibility group (IncI2, IncX4 and IncP1). • One mcr-1- bearing isolate EC09 was susceptible to colistin with MIC of 0.5 mg/L. • EC09 demonstrated lowest transcription level of mcr-1 gene. Colistin resistance mediated by plasmids for their rapid dissemination in Enterobacteriaceae is alarming. We aimed to characterize the genetic features of mcr-1 gene as well as the role of promoters in gene expression and levels of colistin resistance among clinical isolates of Enterobacteriaceae. Methods: Clinical isolates of Enterobacteriaceae were collected in thirteen cities in China and screened for mcr-1 gene using polymerase chain reaction (PCR) amplification and sequencing. Antimicrobial susceptibility testing, transformation assay and plasmid sequencing, quantitative real-time PCR were performed for mcr -1-positive isolates. Promoter-probe vector pKK232-8 was utilized to assess the activity of the mcr-1 promoters. This study identified the mcr-1 gene in 15 clinical isolates of Enterobacteriaceae, among which 14 were resistant to colistin, with MICs of 4–8 mg/L, while one mcr-1- bearing isolate EC09 was susceptible to colistin, with an MIC of 0.5 mg/L. Moreover, mcr-1 -harbouring plasmids from 10 clinical isolates were transferrable via transformation and belonged to different incompatibility groups (IncI2 and IncX4). Plasmid pEC09 failed to transform and belonged to IncP1. A genetic structure containing the mcr-1-pap2 element was detected in these plasmids. EC09 demonstrated the lowest transcription level of mcr-1 gene, as determined by quantitative real-time PCR, which was in accordance with its susceptibility to colistin. Furthermore, the promoter activity of mcr-1 in pEC09 was the lowest, as determined by promoter-probe vector pKK232-8. Promoter variations were associated with expression of the mcr-1 gene and ultimately the levels of colistin resistance. [ABSTRACT FROM AUTHOR]