Seven in absentia (SINA) protein is one subgroup of ubiquitin ligases possessing 
an N-terminal cysteine-rich really interesting new gene (RING) domain, two 
zinc-finger motifs, and a C-terminal domain responsible for substrate-binding 
and dimerization. In tomato (Solanum lycopersicum), the SINA gene family has six 
members, and we characterize in this study all tomato SINA (SlSINA) genes and 
the gene products. Our results show that SlSINA genes are differentially 
regulated in leaf, bud, stem, flower, and root. All SlSINA proteins possess 
RING-dependent E3 ubiquitin ligase activity, exhibiting similar specificity 
towards the E2 ubiquitin-conjugating enzyme. SlSINA1/3/4/5/6 are localized in 
both cytoplasm and nucleus, whereas SlSINA2 is exclusively localized in the 
nucleus. Moreover, all SlSINAs can interact with each other for homo- or 
hetero-dimerization. The functionality of SlSINA proteins has been investigated. 
SlSINA4 plays a positive role in defense signalling, as manifested by 
elicitation of E3-dependent hypersensitive response-like cell death; the other 
SlSINAs are negative regulator and capable to suppress hypersensitive response 
cell death. Transgenic tomato plants overexpressing SlSINA2 exhibit pale-green 
leaf phenotype, suggesting SlSINA2 regulates chlorophyll level in plant cells, 
whereas transgenic tomato plants overexpressing SlSINA5 have altered floral 
structure with exserted stigma, implicating SlSINA5 plays a role in flower 
development.