Monosomy of a specific chromosome determines L-sorbose utilization: a novel regulatory mechanism in Candida albicans

Abstract

We report the identification of the gene, SOU1, required for L-sorbose assimilation in Candida albicans. The level of the expression of SOU1 is determined by the copy number of chromosome III (also denoted chromosome 5), such that monosomic strains assimilate L-sorbose, whereas disomic strains do not, in spite of the fact that SOU1 is not on this chromosome. We suggest that C. albicans contains a resource of potentially beneficial genes that are activated by changes in chromosome number, and that this elaborate mechanism regulates the utilization of food supplies and possibly other important functions, thus representing a novel general means for regulating gene expression in microbes.

Figure 1

Physical map of the SOU1 region showing SOU2 and VMA8 flanking genes; the C-rich 40-bp poly(C) segment; P1, P2, and P3 primers used to generate PCR fragments; SOU1 probe used in Southern and Northern blot analysis; and a portion of the pCA7 plasmid used for disrupting SOU1.

Figure 2

Nucleotide sequence of the SOU1 gene and deduced amino acid sequence of the corresponding Sou1p. The residues conserved in the short-chain alcohol dehydrogenase family are underlined. The C-rich domain and the putative TATA elements are underlined in the 5′ untranslated region. The Sou2p sequence is shown below the Sou1p sequence, where identical residues are depicted by dots.

Figure 3

(A) Southern blot analysis of DNA digestion with BglII and hybridization with the SOU1 probe of the following strains: CAF4–2, the normal SOU1/SOU1 strain; CA18, the singly disrupted SOU1/sou1-Δ∷hisG-URA3-hisG strain; CA19, the singly disrupted SOU1/sou1-Δ∷hisG strain; CA20, the doubly disrupted sou1-Δ∷hisG/sou1-Δ∷hisG-URA3-hisG strain; and CA21, the doubly disrupted sou1-Δ∷hisG/sou1-Δ∷hisG strain. (B) Comparative growth of CAF4–2 and CA19 and CA21 on glucose medium (SD plus uridine) for 6 days at 37°C and (C) on l-sorbose medium (plus uridine) for 14 days at 37°C. Each strain was spotted as serial 1/10 dilutions, from left to right, starting from an initial concentration of 10⁶ cells per ml.

Figure 4

(A) Schematic representation of the electrophoretic karyotypes from 3153A and CAF4–2 showing the positions of the SOU1 gene (▴ and ▾). Orthogonal-field-alternation gel electrophoresis separation of chromosomes of two typical representative sequential series of strains derived from the parental strains (B) 3153A and (C) CAF4–2. Orthogonal-field-alternation gel electrophoresis. Lanes: 1, 3153A (Sou⁻); 2, Sor55 (Sou⁺); 3, Sor55–1 (Sou⁻); 4, Sor55–1-1 (Sou⁺); 5, CAF4–2 (Sou⁻); 6, Sor19 (Sou⁺); 7, Sor19–1 (Sou⁻); 8, Sor19–1-1 (Sou⁺). Conditions of separation in these two representative gels were selected for the best resolution of either the smallest or the smallest and middle-sized groups of chromosomes (21). The gels showing precise separations of the other chromosomes of these strains are not presented. (D) Schematic representation of chromosomes III from the Sou⁺ and Sou⁻ strains serially derived from 3153A and CAF4–2 showing the hypothetical negative regulator CSU1 gene (•).

Figure 5

(A) Expression of SOU1 as determined by Northern blot analysis with a SOU1 probe and with the following strains. Lanes: 1 and 2, parental strain 3153A (Sou⁻); 3, Sor52 (Sou⁺); 4, Sor53 (Sou⁺); 5, Sor52–1 (Sou⁻); and 6, Sor53–1 (Sou⁻). (B) Relative SOU1 mRNA levels were estimated by comparing the hybridization signals with the ethidium bromide fluorescence intensities of the rRNA bands.