Suppression of pea nuclear topoisomerase I enzyme activity by pea PCNA

PCNA interacts with Indian mung bean yellow mosaic virus rep and downregulates Rep activity

Proliferative cell nuclear antigen (PCNA), a conserved plant protein as well as an important replication factor, is induced in response to geminivirus infection in the resting cells of the phloem tissues. The biochemical role of PCNA in rolling circle replication (RCR) of geminivirus DNA has not been explored in detail. The initiation of RCR of the bipartite genome of a geminivirus, Indian mung bean yellow mosaic virus (IMYMV), is mainly controlled by viral protein Rep (or AL1 or AC1). The role of host PCNA in RCR of IMYMV was revealed by studying the physical and functional interactions between recombinant PCNA and recombinant IMYMV Rep. Pea nuclear PCNA as well as recombinant pea PCNA showed binding to recombinant Rep in experiments involving both affinity chromatography and yeast two-hybrid approaches. The contacting amino acid residues of PCNA seemed to be present throughout a wide region of the trimeric protein, while those of Rep appeared to be localized only in the middle part of the protein. The site-specific nicking-closing activity and the ATPase function of IMYMV Rep were impaired by PCNA. These observations lead to interesting speculations about the control of viral RCR and dynamic profiles of protein-protein interactions at the RCR origin of the geminiviruses.

Plant molecular biology and biotechnology research in the post-recombinant DNA era

After the beginning of the recombinant DNA era in the mid-1970s, researchers in India started to make use of the new technology to understand the structure of plant genes and regulation of their expression. The outcome started to appear in print in early the 1980s and genes for histones, tubulin, photosynthetic membrane proteins, phototransduction components, organelles and those regulated differentially by developmental and extrinsic signals were sequenced and characterized. Some genes of biotechnological importance like those encoding an interesting seed protein and the enzyme glyoxalase were also isolated. While work on the characterization of genome structure and organization was started quite early, it remained largely focused on the identification of DNA markers and genetic variability. In this context, the work on mustard, rice and wheat is worth mentioning. In the year 2000, India became a member of the international consortium to sequence entire rice genome. Several laboratories have also given attention to regulated expression of plastid and nuclear genes as well as to isolate target-specific promoters or design promoters with improved potential. Simultaneously, transgenic systems for crops like mustard, rice, wheat, cotton, legumes and several vegetables have been established. More recently, genes of agronomic importance like those for insect resistance, abiotic stress tolerance, nutritional improvement and male sterility, isolated in India or abroad, have been utilized for raising transgenics for crop improvement. Some of these transgenics have already shown their potential in containment facility or limited field trials conducted under the stipulated guidelines. Plant molecular biology and biotechnology are thus clearly poised to make an impact on research in basic biology and agriculture in the near future.

Molecular characterization of the Rep protein of the blackgram isolate of Indian mungbean yellow mosaic virus

The complete nucleotide sequence of the blackgram isolate of mungbean yellow mosaic virus, IMYMV-Bg, which infects legumes in India, was determined and compared at the amino acid level with those of other whitefly-transmitted geminiviruses. The genome organization of IMYMV-Bg was similar to that of the begomoviruses. A unique feature of the genome organization was the sequence divergence of the common region (CR) between DNA-A and DNA-B. In order to understand the mechanism of viral DNA replication, the replication initiator protein, Rep, of IMYMV-Bg was overexpressed in E. coli. The recombinant and refolded Rep bound to CR-sequences of IMYMV-Bg in a specific manner. In this study, evidence is presented for ATP-upregulated cleavage function and ATP-mediated conformational change of Rep. It is hypothesized that, although ATP is not required for cleavage, ATP-mediated conformational changes may result in better access of Rep to the DNA-cleavage site. Evidence is also presented for a site-specific topoisomerase function of Rep, which has not been demonstrated before. The Rep protein can be classified as a type-I topoisomerase because of its nicking activity and sensitivity towards camptothecin, a topoisomerase type-I inhibitor.