THE CHLORELLA HEXOSE H+ SYMPORTER IS A USEFUL SELECTABLE MARKER AND BIOCHEMICAL REAGENT WHEN EXPRESSED IN VOLVOX

The multicellular obligately photoautotrophic alga Volvox is composed of only two types of cells, somatic and reproductive, Therefore, Volvo x provides the simplest model system for the study of multicellularity . Metabolic labeling experiments using radioactive precursors are cruc ial for the detection of stage- and cell-type-specific proteins, glyco proteins, lipids, and carbohydrates. However, wild-type Volvox lacks i mport systems for sugars or amino acids. To circumvent this problem, t he hexose/H+ symporter (HUP1) gene from the unicellular alga Chlorella was placed under the control of the constitutive Volvox beta-tubulin promoter. The corresponding transgenic Volvox strain synthesized the s ugar transporter in a functional state and was able to efficiently inc orporate C-14 from labeled glucose or glucosamine. Sensitivity toward the toxic glucose/mannose analogue 2-deoxyglucose increased by orders of magnitude in transformants. Thus we report the successful transform ation of Volvox with a gene of heterologous origin. The chimeric gene may be selected for in either a positive or a negative manner, because transformants exhibit both prolonged survival in the dark in the pres ence of glucose and greatly increased sensitivity to the toxic sugar 2 -deoxyglucose. The former trait may make the gene useful as a dominant selectable marker for use in transformation studies, whereas the latt er trait may make it useful in development of a gene-targeting system.