Histone H3-lysine(9) (H3K9) trimethyltransferase gene Su(var) 3-9 was cloned and 
identified in three Spodoptera insects, Spodopterafrugiperda (S. frugiperda), S. 
exigua and S. litura. Sequence analysis showed that Spodoptera Su(var) 3-9 is 
highly conserved evolutionarily. Su(var) 3-9 protein was found to be localized 
in the nucleus in Sf9 cells, and interact with histone H3, and the 
heterochromatin protein 1a (HP1a) and HP1b. A dose-dependent enzymatic activity 
was found at both 27 °C and 37 °C in vitro, with higher activity at 27 °C. 
Addition of specific inhibitor chaetocin resulted in decreased histone 
methylation level and host chromatin relaxation. In contrast, overexpression of 
Su(var) 3-9 caused increased histone methylation level and cellular genome 
compaction. In AcMNV-infected Sf9 cells, the transcription of Su(var) 3-9 
increased at late time of infection, although the mRNA levels of most cellular 
genes decreased. Pre-treatment of Sf9 cells with chaetocin speeded up viral DNA 
replication, and increased the transcription level of a variety of virus genes, 
whereas in Sf9 cells pre-transformed with Su(var) 3-9 expression vector, viral 
DNA replication slow down slightly. These findings suggest that Su(var) 3-9 
might participate in the viral genes expression an genome replication repression 
during AcMNPV infection. It provided a new insight for the understanding 
virus-host interaction mechanism.