STRUCTURE AND EXPRESSION OF A GROE-HOMOLOGOUS OPERON OF A MACRONUCLEUS-SPECIFIC SYMBIONT HOLOSPORA-OBTUSA OF THE CILIATE PARAMECIUM-CAUDATUM

The reproductive form of a macronucleus-specific symbiont Holospora ob tusa, when harbored by the macronucleus of the ciliate Paramecium caud atum, selectively synthesized a 63-kDa protein which is immunologicall y related to GroEL, or HSP60, of Escherichia coil. Heat shock treatmen t of isolated cells of the reproductive and infectious form of the bac terium also induced the synthesis of the GroEL homolog. Immunoblotting showed that the amount of this protein per cell, whether the reproduc tive or infectious form, is roughly constant. Cloning and sequencing o f a gene coding for the GroEL homolog suggested that the protein is 55 .2% identical to GroEL of E. coli at the amino acid sequence level, an d that the gene is preceded by an open reading frame which encodes a p rotein 39.6% identical to GroES of E. coli. Northern blot hybridizatio n showed that the groEL homologous gene is highly expressed in the rep roductive form, but only in a trace amount in the intermediate and inf ectious form. Immunoelectron microscopy revealed that the GroEL homolo g is localized in the cytoplasm of the reproductive and infectious for m.