Characterization of a heat-shock-inducible hsp70 gene of the green alga Volvox carteri

The green alga Volvox carteri possesses several thousand cells, but just two cell types: large reproductive cells called gonidia, and small, biflagellate somatic cells. Gonidia are derived from large precursor cells that are created during embryogenesis by asymmetric cell divisions. The J domain protein GlsA (Gonidialess A) is required for these asymmetric divisions and is believed to function with an Hsp70 partner. As a first step toward identifying this partner, we cloned and characterized V. carteri hsp70A, which is orthologous to HSP70A of the related alga Chlamydomonas reinhardtii. Like HSP70A, V. carteri hsp70A contains multiple heat shock elements (HSEs) and is highly inducible by heat shock. Consistent with these properties, Volvox transformants that harbor a glsA antisense transgene that is driven by an hsp70A promoter fragment express Gls phenotypes that are temperature-dependent. hsp70A appears to be the only gene in the genome that encodes a cytoplasmic Hsp70, so we conclude that Hsp70A is clearly the best candidate to be the chaperone that participates with GlsA in asymmetric cell division.