Flocculation of hyphae is associated with a deletion in the putative CaHK1two-component histidine kinase gene from Candida albicans

In Candida albicans three putative histidine kinase genes have been describ ed thus far, including CaSLN1, CaNIK1/COS1 and CaHK1. The encoded proteins for C. albicans, CaSln1p and CaNik1p, which are similar to Sln1p from Sacch aromyces cerevisiae and Nik-1 from Neurospora crassa, seem to function in o smoregulation and morphogenesis, respectively. Recently, the isolation of C aHK1, a putative histidine kinase gene from C. albicans has been reported. In addition to the histidine and aspartyl domains located at its C-terminus as previously described, it is shown here that the N-terminal domain of Ca hk1p contains a P-loop motif and a sequence which shows significant homolog y with the seven C-terminal domains of serine/threonine kinases. The Ser/Th r-homologous domains of Cahk1p could, in fact, correspond to its sensor seq uence. CaHK1 has been mapped to chromosome 2 and gene deletion studies were undertaken to understand its function. Delta cahk1 mutants are phenotypica lly different from any other histidine kinase mutants thus far described ei ther in C. albicans or in any other yeast or filamentous fungus. This study demonstrates that Delta cahk1 mutants flocculate extensively in a gene-dos age-dependent manner under conditions which induce germ-tube formation, suc h as growth in medium 199 (pH 7.5). The flocculation occurs by an interacti on along the hyphal surfaces, probably because of the altered expression of one or more hyphal-cell-surface components in the Delta cahk1 mutants. The se results indicate that CaHK1 could be involved in regulating their expres sion.