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The wide applications of gold nanoparticles in fields such as electronics and biomedicine (targeted-drug delivery) have caught the interest of many researchers. In this study, Serratia marcescens NBL 1001 was screened for its ability to synthesize gold nanoparticles (AuNP). Colorimetric change from light yellow to purple, 24 h after the supplementation of 1mM tetrachloroaric acid (HAuCl4), indicated production of AuNP. UV-Vis scanning spectrophotometry of bacterial suspension, cell-free supernatant and reconstituted cell pellets of S. marcescens NBL1001 showed absorbance peaks characteristic of AuNP (500-600 nm) at 554 nm, 556 nm and 550 nm respectively. Gold nanoparticles with sizes ranging from 11.78 to 46. 03 nm with mean size of 25.28 nm (n = 30) were observed under the scanning electron microscope and the presence of elemental gold (Au0) was confirmed by energy dispersive X-ray spectroscopy.

Summary. A survey of rice fields during the main growing seasons in 81 locations from 21 districts of the Southern Terai region of Nepal indicated that rice tungro was primarily restricted to the Hardinath (Janakpur) and Parwanipur (Bara) regions. The tungro incidence in Hardinath ranged from 17% to 51% and in Parwanipur from 6% to 61% causing about 89% grain yield loss in Hardinath. Both rice tungro bacilliform badnavirus (RTBV) and rice tungro spherical picornavirus (RTSV) were found in tungro isolates collected from Hardinath and Parwanipur. These isolates were transmitted by Nephotettix virescens and leaf extracts reacted to antisera against RTBV and RTSV. In a dot blot hybridisation assay, leaf extracts of 12 weed species collected from the tungro-affected area in Hardinath and Parwanipur also reacted with RTBV DNA probes. On mass inoculation of 15 popular rice cultivars most became more than 50% infected and only cv. Radha 9 had low (22.2%) infection. RTBV DNA and the coat protein region of RTSV from the Hardinath isolate were cloned and partially characterised. A comparative analyses by restriction endonuclease digestion, cross hybridisation, the polymerase chain reaction and partial sequencing indicated that the Nepalese RTBV DNA clone and the cDNA clones of the RTSV RNA were more similar to the various tungro isolates from the Indian subcontinent than to those from the Philippines.

Restriction fragment length polymorphisms (RFLPs) were found in 27 full genome length clones from a glasshouse isolate of rice tungro bacilliform pararetrovirus (RTBV) from the International Rice Research Institute (IRRI), the Philippines and from clones from 5 field isolates from different parts of the Philippines. There was much less variation between the IRRI clones than between the field isolate clones. The RFLPs were due to single base changes and represented about 10% of the potential sites. Sequencing across the region between nt 7772 and 7989 confirmed that the field isolates differed from the published sequence more than did the IRRI clones. The most common substitutions were GA, AG and TC. Sequence heterogeneity was also noted in PCR products from RTBV DNA from the isolates. These observations are discussed in relation to the quasispecies population concept of viruses.