Microscopy and biology of Uronema gigas, a filamentous eucaryotic green alga, and its associated tailed virus-like particle

Abstract

A polyhedral virus-like particle (VLP) 390 nm in diameter was present in large numbers in some cells of two cultures of the filamentous eucaryotic green alga Uronema gigas. Cells which contained VLPs lacked nuclear membranes and were found only in germlings or young filaments. Extracellular particles of similar size and identical ultrastructure accumulated slowly over a 2-month period as the alga grew in liquid culture. Capsid size and appearance of partially purified VLPs was altered in vitro by fixation and pH. A 1-μm-long tail-like appendage with a central swelling was observed attached to some particles in vitro and was also detected in situ. VLPs accumulated to relatively high concentrations in some single-germling subcultures, but were hard to detect in others. No subcultures were obtained which failed to produce VLPs. A correlation was detected between high VLP productivity and debilitated growth. VLP productivity could be increased in a high yielding subculture by a heat shock treatment during the first 7 hr of a 2-week growth cycle. Pronase digestion of thin sections of fixed and embedded partially purified VLPs indicated that protein was a major component of the capsid shell and contents. DNAse treatments of thin sections and X-ray microanalysis in the scanning electron microscope suggested VLP-associated DNA.