We have analysed function of 2500 C. elegans genes by an RNAi-by-soaking method using a non-redundant cDNA set. Among them, some caused sterility but no somatic phenotypes. Two HMG box genes belonging to distinct HMG subfamilies, namely
hmg-3 (C32F10.5) and
hmg-5 (F45E4.9), both of which caused similar underproliferated germline phenotypes in F1 generation. In order to analyse the function of each HMG gene at the post-embryonic stage, L1 larvae were soaked in dsRNA solution (RNAi L1 ). About 51% of
hmg-3(RNAi L1 ) worms showed sterility with few germ cells, but no
hmg-5(RNAi L1 ) worm showed sterility. These results imply that
hmg-3 is required for germline proliferation in the postembryonic stage, whereas
hmg-5 is required only in embryogenesis. The
hmg-3 gene has one paralog,
hmg-4 (T20B12.8), which shows 77% identity. Unlike
hmg-3(RNAi) ,
hmg-4(RNAi) caused larval lethality. The affected F1 progeny were paralyzed and died at the L1-2 stage, but no other morphological abnormalities were observed. In addition, disruption of both
hmg-3 and
hmg-4 by RNAi resulted in synthetic embryonic lethality. The arrested embryos showed gut and muscle differentiation, but they failed to undergo morphogenesis. 4D time-lapse recording showed that cell division started delaying around 2hr after pronuclei fusion, and seemed to arrest before the onset of morphogenesis. They were apparently normal in early embryogenesis. Therefore, these two genes may be functionally redundant in embryogenesis, and they may function independently in germ and soma after embryogenesis. HMG-3 and HMG-4 are members of the structure-specific DNA recognition protein (SSRP) family, and HMG-5 has a weak similarity to human mitochondrial transcription factor 1. In vertebrates, SSRP family proteins are known to be involved in various processes, such as DNA replication, transcription and V(D)J recombination. For further analysis of the specific function of each gene, we plan to isolate deletion mutants of the hmg genes.