טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRoitman Sheila
SubjectBiodiversity of Marine Viruses Carrying Photosystem I Genes
DepartmentDepartment of Biology
Supervisor Professor Beja Oded
Full Thesis textFull thesis text - English Version


Abstract

Marine photosynthesis is largely driven by cyanobacteria, mainly Synechococcus and Prochlorococcus. Cyanophages can alter cyanobacterial populations, affecting the global carbon cycle and oxygen supply. During the infection some phages express unique metabolic genes which might interfere with the host ongoing metabolism. Genes encoding for photosystem II (PSII) and I (PSI) reaction center proteins are found in cyanophages and are believed to increase their fitness, possibly by being involved in the cyanobacterial photosynthesis. PSI genes in phages are arranged in two known gene cassettes, psaJF→C→A→B→K→E→D and psaD→C→A→B. The shared genes between these cassettes and their encoded proteins are distinguished by %G and protein sequence, respectively. The data on the psaD→C→A→B gene organization is scarce and was obtained from two Global Ocean Sampling (GOS) stations only, in the Pacific and Indian Oceans. We wanted to expand our knowledge regarding the biodiversity of the phages carrying PSI genes and understand the differences between both viral PSI cassettes and their possible hosts. We conducted a targeted screening search using PCR on viral concentrates and a comprehensive bioinformatic search in six metagenomic datasets. We surveyed 370 marine stations, resulting in new positive stations for viral-PSI genes in the Pacific, Indian and Atlantic Oceans. The stations containing the psaD→C→A→B gene arrangement (as well as the longer gene arrangement) are confined to a strip along the equator (± 30o). According to our metagenomic findings, viral photosynthetic genes are found solely in the two previously described cassettes and might belong to phages from the Myoviridae family. In addition, we succeeded in filling the partially assembled psaD→C→A→B cassette sequence. Protein structure modeling of both viral PsaA show structural differences, the psaD→C→A→B PsaA protein being more similar to Synechococcus. We propose that phages carrying PSI genes in the psaD→C→A→B cassette might infect Synechococcus or low light (LL) clade IV Prochlorococcus which have a similar protein structure and inhabit the oceans between 30N, 30S.