Abstract: To identify the genes related to the regulation of early flowering at low temperature in tobacco, two full-length cDNAs encoding different E3 ubiquitin ligase genes (NtHOS1a and NtHOS1b) were cloned from Nicotiana tabacum by bioinformatics, RT-PCR and SMART RACE technologies. GenBank accession numbers of NtHOS1a and NtHOS1b were KU558689 and KU558690, respectively. The putative full-length cDNA sequences of NtHOS1a and NtHOS1b were 3 599 bp and 3 578 bp, and these two genes encoded the predicted polypeptide of 963 and 954 amino acids, respectively. Amino acids identity between NtHOS1a and NtHOS1b was 95%. Putative polypeptide in the two genes matched Arabidopsis thaliana HOS1(NP_181511) polypeptide sequence with the identity of 51% and 50%, respectively. The two proteins shared highly conserved N-terminal RING domains of E3 ubiquitin ligases. qRT-PCR showed that under normal growing condition, NtHOS1a and NtHOS1b transcripts accumulated to a high level in root, stem and leaves. After 10 days of 12 ℃ treatment, the expression levels of the two genes were decreased to different degrees, this suggested that NtHOS1a and NtHOS1b expressions in tobacco root, stem and leaves were negatively regulated by low temperature.