# Biotin Auxotrophy and Biotin Enhanced Germ Tube Formation in _Candida albicans_
## 4 Discussion  
There are a great many chemical and physical signals for initiating germ tube formation and hyphal growth in _C. albicans_. These signals are important because yeast-mycelial dimorphism was the first virulence trait recognized in pathogenic fungi and as such it is somewhat perplexing that the actual germ tube triggers are still uncertain for [PERSON]'s medium [2] and serum, two of the growth media most commonly used to get filamentous growth. This paper examined the physiological consequences of widely diverse levels of biotin (0-4 uM) on _C. albicans_ with a view towards establishing high biotin as a factor that enhances germ tube formation (GTF). Biotin may eventually be shown to be a trigger for GTF, but at present, our data only prove that it is an enhancer of GTF. Three well studied conditions for GTF were shown to be biotin dependent: [PERSON]'s medium, 1% serum, and GPP (Figures 2-5). In response to high biotin (4 uM), _C. albicans_ accumulates internal biotin reserves, forms biotinylated histones [11], and initiates germ tubes and hyphal growth. The hyphae produced during high biotin growth are physiologically normal in the sense that only 1-3 uM famesol is needed to block GTF in [PERSON]'s medium or RPMI 1640 for both _C. albicans_ A72 and SC5314 [32].  
_C. albicans_, like many yeasts, is a biotin auxotroph with an ability to accumulate internal, covalently attached biotin as a reserve for subsequent growth when biotin is limiting. Because of these internal reserves of biotin in _C. albicans_, claims for growth of _C. albicans_ strain B 311-10 in minimal synthetic biotin-free medium [47] must be treated with caution because they used only a single passage of 30 h on the biotin-free medium. The rationale for _C. albicans_ having an incomplete biosynthetic pathway for biotin (i.e., being able to utilize some biotin precursors as well biotin) is provided by the observation of [PERSON] et al. [33] that biotin only accounts for half of the total avidin-binding substances present in human serum.  
Despite being a biotin auxotroph, _C. albicans_ SC5314 and WO-1 contain distinct biotin clusters including _BIO2_, _BIO3_, _BIO4_, and _BIO5_[38]. With regard to pathogenicity, the implication is that biotin or its precursors desthiobiotin and KAPA are available in the blood or another location where infection is occurring. [PERSON] et al. [48] catalogued the _C. albicans_ genes which were significantly upregulated during infection; both _BIO2_ and _BIO4_ were upregulated ca. 2.5-fold. Similarly, _BIO2_ was upregulated ca. 107-fold on shifting from low iron to high iron growth conditions [49]. Figure 1 illustrates biotin biosynthesis in _C. albicans_. For both yeasts, only _BIO2_ is predicted to be mitochondrion-localized. Presumably _BIO2_ has to remain mitochondrial because, as in _A. thaliana_[50], the biotin sulfur atom is obtained from an iron/sulfur cluster protein. _C. albicans_ does not contain _BIO1_ (pimeloyl-CoA synthase) or _BIO6_, the alternate method of synthesizing KAPA found in biotin prototrophic sake yeasts [51]. We anticipate that the isolation and characterization of mutants unable to respond to high biotin will elucidate biotin's mode of action in stimulating GTF.