Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Molecular studies of chlorella viruses
Abstract
Plaque-forming viruses of Chlorella sp. strain NC64A are common in fresh water collected in the People's Republic of China. Seven of these Chinese viruses were examined in detail and compared with the chlorella viruses previously isolated in the United States. Although the Chinese viruses were morphologically and genetically similar to the American viruses, they were placed into five new classes on the basis of plaque morphology, reaction with antibodies, resistance of the virus DNAs to restriction endonucleases and most importantly the nature and abundance of methylated bases in their genomic DNAs. The prototype chlorella virus PBCV-1 genome is a linear nonpermuted 333 kbp dsDNA molecule with covalently closed hairpin termini. The termini (minus the hairpin) contain identical inverted terminal repeat sequences (ITR) of at least 2,185 bases after which the sequences diverge. The ITR contained two small open reading frames, but neither of them are transcribed during PBCV-1 replication, and several small direct repeats. Twenty-nine of thirty-six other chlorella viruses tested hybridized to PBCV-1 ITR. Of eleven selected virus DNAs, which hybridized to the PBCV-1 ITR, the probe hybridized to two distinct fragments, which presumably are the terminal fragments of these virus DNAs. The chlorella virus PBCV-1 contains an open reading frame, named P17-ORF4, at the right hand terminal of its genome which lies immediately upstream of the 2.2 kbp ITR region. P17-ORF4 differs from a cytosine methyltransferase gene M.Cvijl, present in chlorella virus IL-3A by eight amino acids, and does not have M.Cvijl activity, and so appears to be a pseudogene. A key amino acid change at position 188 from glutamine to lysine is responsible for the loss of enzyme activity. The other altered seven amino acids do not have significant effect. From predicted secondary structures, we propose that the Gln$\sp{188}$ to Lys$\sp{188}$ change prevents the Pro-Cys active site of M.CviJl from interacting with the DNA target recognition domain of the enzyme. Pseudogenes for other methylation, possibly restriction, enzymes may be common in chlorella viruses.
Subject Area
Molecular biology|Microbiology|Plant pathology
Recommended Citation
Zhang, Yanping, "Molecular studies of chlorella viruses" (1991). ETD collection for University of Nebraska-Lincoln. AAI9219396.
https://digitalcommons.unl.edu/dissertations/AAI9219396