@article{egelkrout_mariconti_settlage_cella_robertson_hanley-bowdoin_2002, title={Two E2F elements regulate the proliferating cell nuclear antigen promoter differently during leaf development}, volume={14}, ISSN={["1040-4651"]}, DOI={10.1105/tpc.006403}, abstractNote={E2F transcription factors regulate genes expressed at the G1/S boundary of the cell division cycle in higher eukaryotes. Although animal E2F proteins and their target promoters have been studied extensively, little is known about how these factors regulate plant promoters. An earlier study identified two E2F consensus binding sites in the promoter of a Nicotiana benthamiana gene encoding proliferating cell nuclear antigen (PCNA) and showed that the proximal element (E2F2) is required for the full repression of PCNA expression in mature leaves. In this study, we examined the distal element (E2F1) and how it interacts with the E2F2 site to regulate the PCNA promoter. Gel shift assays using plant nuclear extracts or purified Arabidopsis E2F and DP proteins showed that different complexes bind to the two E2F sites. Mutation of the E2F1 site or both sites differentially altered PCNA promoter function in transgenic plants. As reported previously for the E2F2 mutation, the E2F1 and E2F1+2 mutations partially relieved the repression of the PCNA promoter in mature leaves. In young tissues, the E2F1 mutation resulted in a threefold reduction in PCNA promoter activity, whereas the E2F1+2 mutation had no detectable effect. The activity of E2F1+2 mutants was indistinguishable from that of E2F2 mutants. These results demonstrate that both E2F elements contribute to the repression of the PCNA promoter in mature leaves, whereas the E2F1 site counters the repression activity of the E2F2 element in young leaves.}, number={12}, journal={PLANT CELL}, author={Egelkrout, EM and Mariconti, L and Settlage, SB and Cella, R and Robertson, D and Hanley-Bowdoin, L}, year={2002}, month={Dec}, pages={3225–3236} }
 @article{egelkrout_robertson_hanley-bowdoin_2001, title={Proliferating cell nuclear antigen transcription is repressed through an E2F consensus element and activated by geminivirus infection in mature leaves}, volume={13}, ISSN={["1531-298X"]}, DOI={10.1105/tpc.13.6.1437}, abstractNote={The geminivirus tomato golden mosaic virus (TGMV) amplifies its DNA genome in differentiated plant cells that lack detectable levels of DNA replication enzymes. Earlier studies showed that TGMV induces the accumulation of proliferating cell nuclear antigen (PCNA), the processivity factor for DNA polymerase delta, in mature cells of Nicotiana benthamiana. We sought to determine if PCNA protein accumulation reflects transcriptional activation of the host gene. RNA gel blot analysis detected an approximately 1200-nucleotide PCNA transcript in young leaves. The same RNA was found in mature leaves of infected but not healthy plants. Reporter gene analysis showed that a 633-bp promoter fragment of the N. benthamiana PCNA gene supports high levels of expression in cultured cells and in young but not mature leaves of healthy transgenic plants. In contrast, PCNA promoter activity was detected in both young and mature leaves of TGMV-infected plants. Developmental studies established a strong relationship between symptom severity, viral DNA accumulation, PCNA promoter activity, and endogenous PCNA mRNA levels. Mutation of an E2F consensus element in the PCNA promoter had no effect on its activity in young leaves but increased transcription in healthy mature leaves. Unlike the wild-type PCNA promoter, TGMV infection had no detectable effect on the activity of the mutant E2F promoter. Together, these results demonstrate that geminivirus infection induces the accumulation of a host replication factor by activating transcription of its gene in mature tissues, most likely by overcoming E2F-mediated repression.}, number={6}, journal={PLANT CELL}, author={Egelkrout, EM and Robertson, D and Hanley-Bowdoin, L}, year={2001}, month={Jun}, pages={1437–1452} }
 @article{peele_jordan_muangsan_turnage_egelkrout_eagle_hanley-bowdoin_robertson_2001, title={Silencing of a meristematic gene using geminivirus-derived vectors}, volume={27}, ISSN={["0960-7412"]}, DOI={10.1046/j.1365-313x.2001.01080.x}, abstractNote={Geminiviruses are DNA viruses that replicate and transcribe their genes in plant nuclei. They are ideal vectors for understanding plant gene function because of their ability to cause systemic silencing in new growth and ease of inoculation. We previously demonstrated DNA episome-mediated gene silencing from a bipartite geminivirus in Nicotiana benthamiana. Using an improved vector, we now show that extensive silencing of endogenous genes can be obtained using less than 100 bp of homologous sequence. Concomitant symptom development varied depending upon the target gene and insert size, with larger inserts producing milder symptoms. In situ hybridization of silenced tissue in attenuated infections demonstrated that silencing occurs in cells that lack detectable levels of viral DNA. A mutation confining the virus to vascular tissue produced extensive silencing in mesophyll tissue, further demonstrating that endogenous gene silencing can be separated from viral infection. We also show that two essential genes encoding a subunit of magnesium chelatase and proliferating cell nuclear antigen (PCNA) can be silenced simultaneously from different components of the same viral vector. Immunolocalization of silenced tissue showed that the PCNA protein was down-regulated throughout meristematic tissues. Our results demonstrate that geminivirus-derived vectors can be used to study genes involved in meristem function in intact plants.}, number={4}, journal={PLANT JOURNAL}, author={Peele, C and Jordan, CV and Muangsan, N and Turnage, M and Egelkrout, E and Eagle, P and Hanley-Bowdoin, L and Robertson, D}, year={2001}, month={Aug}, pages={357–366} }