טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLimor Keren
SubjectInsight into the psbA and psbD Photosynthesis Genes of
cyanobacteria and cyanophges
DepartmentDepartment of Biology
Supervisor Professor Oded Beja
Full Thesis textFull thesis text - English Version


Abstract

Cyanophages, viruses infecting cyanobacteria, are classified into three distinct families Podoviridae, Myoviridae and Siphoviridae. Several cultured members of the two former families, Podoviridae and Myoviridae, were recently shown to possess photosynthesis genes upon their genome presumably originating from their hosts, the widely distributed unicellular cyanobacteria of the genus Prochlorococcus and Synechococcus.  Great interest rose in two of the photosynthesis genes, psbA and psbD, encoding for the two core reaction center proteins of Photosystem II, D1 and D2 proteins respectively, concerning their utility once incorporated into genomes of cyanophages.

In the present research a large set of environmental DNA sequences was analyzed to evaluate the extent, the context and the function of the incorporation events of the psbA and psbD genes into genomes of cyanophages. Podoviridae were never found to incorporate the psbD gene while Myoviridae often do. Parallel phylogenetic analyses of Myoviridae psbA and psbD genes suggest a closely related evolutionary process of the two genes. It was also observed that Myoviridae infecting-Prochlorococcus-like tend to cluster the psbA and psbD in close proximity to each other while Myoviridae infecting-Synechococcus-like tend to cluster them further apart. Finding of photosynthesis and other bacterial genes, previously reported on a narrow dataset of cultured cyanophages, underlines their importance and suggests functionality.  Further insight into the two genes revealed that each family of cyanophages or cyanobacterial genus  present conserved modifications of amino acids compositions at specific sites on the D1 and D2 proteins, some of which were formerly suggested to serve as target sites for proteases regulating the turnover rate, suggesting tuning of the repair cycle of Photosystem II. The overall set of data reveals different strategies used by different cyanophages to control their relative hosts.