| Candida albicans is an opportunistic fungal pathogen of humans and is one of the most common causes of nosocomial bloodstream infections. Immunocompromised individuals are at greatest risk for life-threatening candidiasis. Nitrogen is an essential component of all living cells. Some nitrogen sources are more readily metabolized and microorganisms selectively utilize these preferred nitrogen sources in the environment. This prevents unnecessary expression of genes required to transport and metabolize poor nitrogen sources. Nitrogen catabolite repression (NCR) is one of the mechanisms controlling selectivity. Gat1p, a member of the zinc-finger containing GATA-family transcription factors, is a transcriptional activator participating in NCR of Candida albicans. Deletion of gat1 attenuates the virulence of C. albicans in a mouse model of disseminated infection accentuating the importance of NCR in adaptation of C. albicans to host niches. Here we identify Gln3p as a second positive regulator of NCR. Deletion of GLN3 compromised the expression of a subset of NCR-sensitive genes. These included both GAT1-independent (MEP2 and GDH1) and GAT1-dependent (UGA4 ) genes, indicating that Gln3p can activate transcription independently or cooperatively with Gat1p. The growth rate of gln3 deletion mutants, like gat1 mutants, was reduced when isoleucine or tryptophan were provided as the sole nitrogen source, demonstrating that these catabolic pathways are co-regulated by these factors. Expression of GLN3 was independent of nitrogen source, and was negatively regulated by GAT1, a regulatory interaction not seen in other fungi. However, like other fungi, Gln3p activity appears to be regulated by Tor1p (target of rapamycin) since loss of GLN3 conferred resistance to rapamycin. Morphological transition is stimulated by poor nitrogen conditions suggesting that NCR may be central to this behavior. Unexpectedly, deletion of both gat1 and gln3 did not prevent hyphal development suggesting that NCR does not regulate filamentation. However, the involvement of other GATA factors can not be excluded. Deletion of gln3 attenuated the virulence of C. albicans in a mouse model of systemic infection. Global expression analysis by microarray hybridization identified GLN3-dependent genes including some reported virulence determinants, e.g. secreted aspartyl proteinases and Hwp1p. Those genes might account for the attenuation of Deltagln3 mutants. |
