Abstract: The strategies for improving the expression efficiency of foreign genes in cyanobacterial hosts are,so far,focusing on modifying donor DNA elements.This work described here was an attempt to physiologically change host systems and see how they affected the expression efficiency of foreign genes in cyanobacteria.Our previous work presented that filamentous cyanobacterium Anabaena sp.PCC7120 cells induced by high temperature and red light can generate short filaments (S-HT and S-RL,respectively) with photosynthetic activities that has the ability to normally differentiate into vegetable filaments.We transferred here a foreign gene (human tumor necrosis factor-α,hTNF-α) into these short filaments.A shuttle expression vector pKT-TNF harboring human TNF-α gene was constructed and transconjugated into the cells of S-HT,S-RL and the normal vegetative filaments (control) by triparental conjugal mating,respectively.The data showed that the transconjugative efficiencies of pKT TNFs in the S-HT and S-RL were 500%-600% of those in the control cells.The Southern blotting confirmed TNF-α gene was replicated in transconjugative cells of S-HT,S-RT and control.The Western blotting suggested that pKT-TNFs were expressed in these cells.The radioimmunoassay (RIA) showed expression efficiency of pKT-TNFs in the S-HT and S-RL cells were about 400%-500% of those in the control cells.This might provide a novel potential avenue to improve the expression efficiency of foreign genes in filamentous cynobacteria.In addition,we also analyzed the effect of genetic code bias of human TNF-α gene on its expression in Anabaena sp.PCC7120.