Chlorella virus PBCV-1 encodes a functional homospermidine synthase

Sequence analysis of the 330-kb genome of chlorella virus Paramecium bursar ia chlorella virus 1 (PBCV-1) revealed an open reading frame, A237R, that e ncodes a protein with 34% amino acid identity to homospermidine synthase fr om Rhodopseudomonas viridis. Expression of the a237r gene product in Escher ichia coli established that the recombinant enzyme catalyzes the NAD(+)-dep endent formation of homospermidine from two molecules of putrescine. The a2 37r gene is expressed late in PBCV-1 infection. Both uninfected and PBCV-1; infected chlorella, as well as PBCV-1 virions, contain homospermidine, alon g with the more common polyamines putrescine, spermidine, and cadaverine. T he total number of polyamine molecules per virion (similar to 539) is too s mall to significantly neutralize the virus double-stranded DNA (>660,000 nu cleotides). Consequently, the biological significance of the homospermidine synthase gene is unknown. However, the gene is widespread among the chlore lla viruses. To our knowledge, this is the first report of a virus encoding an enzyme involved in polyamine biosynthesis. (C) 1999 Academic Press.