In order to rapidly screen new inhibitors of human DNA topoisomerase I in vitro, the enzyme has been successfully overexpressed in the full-length form (hTopoI) and N-terminal 184 amino acids truncated form (hTopoI△581) in Pichia pastoris by the secretory and intracellular styles. The results showed that the recombinant enzyme containing nuclear localization signals obviously decreased its expression level. If the enzyme was fused to α-factor secretory signal or the AKL triplet amino acids (a C-terminal peroxisomal-targeting signal), its output could be highly improved. The recombinant strain SMD-hTopoI△ (out) was best suited to express and secrete the highly active hTopoI△ 581 with part of them glycosylated (the enzyme activity of the supernatant reached 3.4×10 8Ul-1). SMD-hTopoI (out) could produce and secrete the full length form of hTopoI without glycosylation (the enzyme activity of the supernatant reached 4.3×10 7Ul-1). The yield of the recombinant hTopoI reached 11mgl-1, achieving 9.7% of the total protein in the culture supernatant, while that of the recombinant hTopoI△ 581 reached 58mgl-1, achieving 47.3% of the total protein in the supernatant. Both forms of the recombinant enzyme exhibited the same properties as the natural hTopoI.

Two hundred and forty-two actinomycete strains were isolated from the interior of leaves and roots of healthy and wilting banana plants. Most of them were streptomycetes, Streptomyces griseorubiginosus-like strains were the most frequently isolated strains. Community analysis demonstrated increased actinomycete diversity in wilting leaves compared to that in healthy leaves, similar actinomycete communities were found in wilting and healthy roots. Screening of the isolates for antagonistic activity against Fusarium oxysporum f. sp. cubense revealed that the proportion of antagonistic streptomycetes in healthy roots was higher than that in wilting roots (P<0.01), but no difference was found between antagonistic strains isolated from healthy and wilting leaves. The potential biological control of Panama disease of banana by endophytic streptomycetes, especially Streptomyces griseorubiginosus-like strains was discussed.

The populations of endophytic fungi and actinomycetes from four rice cultivars in the Panyu district (Site 1) and Wushan district (Site 2) in Guangdong province, South China, were studied. The preponderant endophytic fungi and actinomycetes isolated belonged to Fusarium and Streptomyces respectively. The incidence of Streptomycetes griseofuscus ranged from 36.1 to 69% out of all the different rice cultivars from the two sites. It is the commonest population of endophytic actinomycetes, and constituted the greatest part of all the antagonistic communities. The distributions of endophytic fungi and actinomycetes in roots and leaves were different, endophytic fungi from leaves were diverse, some were organ-specific. More diverse endophytic actinomycetes were isolated from roots than from leaves. The endophytic fungi isolated from rice in Site 2 were more diverse than that in Site 1. The diversity of the endophytic actinomycetes, however, was less than that in Site 1. Acid soil in Site 2 is ideal for the growth and colonization of fungi while the alkaline soil in Site 1 is better for the growth and colonization of actinomycetes. The results suggested that differences in the chemical composition of soil could influence the endophytic microbial communities of rice plants. The endophytic fungi and actinomycetes isolated from poor-growing seedlings and susceptible rice cultivars were more abundant than that the disease-resistant counterparts. In the dual culture and activity detection of the metabolites, 41.2% of all the isolated endophytic fungi showed antagonism to rice pathogens. Fifty percent of all the isolated endophytic actinomycetes were antagonistic to those pathogens. The percentage of Streptomyces griseofuscus and hygroscopicus reached 55.4 and 21.4% of all the active actinomycetes.

Aims: To isolate endophytic Streptomyces strains from tomato and examine their antimicrobial activity. Methods: Endophytic Streptomyces strains were isolated using surface-sterilization methods and identified by morphological characteristics. Antimicrobial activities were measured by the agar plate sensitivity method. Antifungal activity in vivo was measured by seedling mortality in infested soils. Results: Twenty-one per cent of endophytic streptomycete isolates produced antibacterial metabolites and 41% produced antifungal metabolites in S medium. Sixty-five per cent of the most frequently isolated strains inhibited the growth of Rhizoctonia solani by the antibiosis assay but only 32% produced metabolites active against R. solani in S medium. Growth promotion and enhanced disease resistance of seedlings inoculated with Streptomyces sp. strain S30 were observed in tomato but not in cucumber seedlings. Conclusions: Endophytic Streptomyces spp. strains were successfully isolated using stringent methods and strain S30 promoted growth and enhanced resistance to R. solani in tomato seedlings. Significance and Impact of the Study: Endophytic streptomycetes showing antifungal activity in vitro and in vivo may indicate the potential for their use as biocontrol agents particularly of R. solani disease of tomato seedlings.