We have investigated the functional relationship between two proteins involved in the photosensory system of the archaeon Halobacterium salinarium: the photoreceptor sensory rhodopsin I (SRI) and the halobacterial transducer rhodopsin I (HtrI), which has been proposed to be the putative signal transducer of SRI, by genomic DNA analysis of two independent SRI negative mutants, Pho81 and D1. Southern and PCR analyses revealed that both strains bear alterations in the 5' flanking region of the gene encoding SRI, sopI. DNA sequence analysis confirmed the occurrence in this region of htrI, the gene encoding the putative transducer protein. PCR and Northern analyses have shown further that sopI and htrI are expressed as a single transcriptional unit, thus explaining the lack of SRI in mutants with a defective htrI. Expression of the cloned sopI under the control of a heterologous promoter did not restore the SRI-dependent photoresponse in the strain Pho81. Moreover, the photocycling rate of the expressed pigment was clearly lower than in wild type. HtrI is therefore essential for SRI function and most likely modulates the photochemical properties of the photoreceptor via direct physical interaction. Finally, reintroduction of both sopI and htrI into Pho81 and D1 restored the SRI photochemistry and its physiological function. Our results provide the first experimental evidence for the functional coupling between SRI and HtrI and corroborate the proposed model in which HtrI acts as the signal transducer of this archaeal seven-helix photoreceptor in a way analogous to the bacterial chemotaxis transducers.