Minireview Genetic Transformation in Aspergilli
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Indian Journal of Biochemistry & Biophysics Vol. 41, October 2004, pp. 205-215 Minireview Genetic transformation in Aspergilli: Tools of the trade V Lakshmi Prabha and N S Punekar* Biotechnology Group, School of Biosciences and Bioengineering, Indian Institute of Technology, Bombay, Mumbai 400 076 India Received 6 May 2004; revised 19 August 2004 DNA-mediated transformation is a powerful tool that allows the introduction of specific genetic changes in an organism. Transformation of Aspergilli, acclaimed for their wide use in the industry, has been possible for about two decades now. Several basic and applied problems related to fungal biology have been addressed using this technique. Nonetheless, new markers and strategies for transformation are still being developed for these filamentous fungi. Different methods and markers that are currently available for the transformation of Aspergilli are summarized here. The review also brings out the importance of these transformation systems in analyzing fungal gene function. Aspects of Aspergillus niger transformation are selectively emphasized. Keywords: Aspergilli, Aspergillus niger, transformation strategies, genetic markers, transformation vectors, gene function Introduction arduous, due to high melanin content of its cell wall; Aspergilli are of immense economic and medicinal and b) excessive clumping of spores occurs during value and a better understanding of their basic germination2. metabolic processes would enable the control and The first report of transformation of a filamentous treatment of pathogenic species and in the industrial fungus appeared in 1979 when DNA was introduced scenario help in rationalized strain improvement1. into protoplasts of Neurospora crassa3. Genetic transformation is an important tool that Transformation of A. nidulans that followed shortly allows one to address questions relating to afterward4 ushered in an era that has seen rapid metabolism and gene expression as well as the advancement in the understanding of these organisms. regulation of these processes. In Aspergilli that Although protoplast transformation remains one of reproduce predominantly through asexual mode it is the most commonly adopted protocols, several other of further value as it provides a means of introducing methods have been developed since then. They are specific genetic changes. i) Agrobacterium-mediated gene transfer; ii) In filamentous fungi, genetic transformation is electroporation of young germlings; iii) particle usually achieved by integration of the introduced bombardment; and iv) DNA uptake by lithium acetate DNA into the genome of the organism. Integration is treated cells. Salient features of these methods are mostly through heterologous recombination; enumerated below. A review of recent literature on homologous recombination in fungi occurs only at the different markers used/available to transform low frequencies. The lower transformation filamentous fungi is presented with an emphasis on efficiencies (in relation to yeast or E. coli) observed in Aspergilli. Although there are a number of patents in filamentous fungi may largely be ascribed to their this general field, this review is largely confined to tough cell wall. Although the problems encountered literature in the public domain barring few exceptions. in transforming Aspergilli are not unique, some of these species are more difficult to transform, than the Entry of incoming DNA others. For instance, in A. niger — a) protoplasting is Transformation of protoplasts ______________ The most routinely used protocol for the *Author for correspondence transformation of filamentous fungi involves the E-mail: [email protected] preparation of protoplasts followed by DNA uptake Tel: 91-22-2576 7775 mediated either by CaCl2/PEG treatment or more Fax: 91-22-2572 3480 recently by electroporation5,6. Protoplasts are prepared Abbreviations: 5-FOA, 5-fluoroorotic acid; GRAS, generally regarded as safe; OMP, orotidine-5’-monophosphate; PCR, from swollen conidia or from young mycelia by polymerase chain reaction; PEG, polyethylene glycol. enzymatic digestion of their cell walls. For 206 INDIAN J. BIOCHEM. BIOPHYS.,VOL. 41, OCTOBER 2004 transformation, protoplasts in osmotic medium interest (gene to be introduced into the fungus) and a containing CaCl2 are incubated with exogenous DNA fungal selectable marker are cloned between the and PEG (aids transformation by bringing the DNA direct repeats in an E. coli-compatible plasmid and and protoplasts together). Highest transformation introduced into Agrobacterium to generate the efficiencies are reported to coincide with germ tube T-DNA, while the necessary virulence functions are emergence in A. nidulans7. provided in trans from a Ti plasmid devoid of T-DNA15. Electroporation Agrobacterium-mediated DNA transfer in Transformation by electroporation involves filamentous fungi is possible with both conidia and 16,17 exposing cells to short duration high voltage electrical protoplasts . The transforming DNA usually pulses in the presence of exogenous DNA. The integrates randomly into the host genome. However, electrical pulses induce a reversible permeabilization this technique has also been used to mediate gene of the plasma membrane and transient pores are targeting by homologous recombination in 18 formed enabling the DNA to enter the cell8. A. awamori . Electroporation was initially used for the transformation of protoplasts as an alternative to the Other approaches 6 CaCl2/PEG method . It was later demonstrated that Transformation of lithium acetate treated cells, germinating conidia could also be transformed using routinely used for yeast and also demonstrated in this technique. Although the first reports of the N. crassa, has not been reported for Aspergilli. procedure included the use of a mild pretreatment However, direct transformation of lysine requiring with β-glucuronidase to weaken the cell wall9, mutants of Aspergillus ochraceus could be achieved transformation of swollen conidia without the need by subjecting the mutant conidia to starvation for pretreatment has been reported for A. niger10, conditions for 15 days and then treating them with A. nidulans11 and A. fumigatus12. DNA from the wild-type strain19. This method nevertheless is not commonly used. Biolistic transformation Particle bombardment as a technique for DNA Comparison of different methods delivery was developed in the late 1980's13. In this The methods described above have their own pros method, the DNA is first precipitated on small (1 μm) and cons. Transformation of protoplasts necessitates gold or tungsten particles, which are accelerated their isolation, which is a laborious procedure towards the target in partial vacuum. After particle requiring great care in experimentation. The penetration, the DNA dissociates from the particles competence achieved by the protoplasts varies vastly and moves to the nucleus, where it gets integrated into with the batch of mycolytic enzyme used. A 100-fold slump in transformation frequency on varying the the genome. A. nidulans is perhaps the only 20 Aspergillus species that has been transformed using batch of enzyme used has been reported . This this method14. This method is generally more useful method is also limited due to by the low and often for the transformation of fungi that are refractory to variable regeneration frequency of protoplasts. other methods. Nonetheless, protoplast transformation remains the most commonly adopted procedure. Electroporation is simpler and provides a good alternative to protoplast Agrobacterium-mediated transformation transformation; it, however, requires a specialized This method exploits the innate virulence apparatus and offers no advantage in terms of (conferred by the Ti plasmid) of Agrobacterium to improved transformation efficiency. Biolistic achieve transformation. The bacterium is co- transformation is expensive and again, requires cultivated with the host to be transformed in the elaborate apparatus. The method is, however, presence of virulence-inducing molecules like important in those fungi that are refractory to other acetosyringone. This leads to the mobilization of transformation protocols. T-DNA, a region of the Ti plasmid flanked by imperfect 24 bp direct repeats. The T-DNA Transformation mediated by Agrobacterium is the subsequently enters the host cell and gets integrated most recent and perhaps the simplest method of all. It into the genome. In the binary system, the gene of is often more effective than the conventional methods PRABHA & PUNEKAR: GENETIC TRANSFORMATION IN ASPERGILLI 207 of protoplast transformation. For instance, use of Nutritional markers Agrobacterium improved the transformation An appropriate mutant recipient strain must be frequency of A. awamori up to 600-fold as compared available to employ nutritional markers. Selection in with protoplast transformation16. A comparison of the case of nutritional markers is based on four different transformation methods has been complementation of a nutritional defect by 22 reported for Aspergillus giganteus. Agrobacterium- introduction of the wild type allele . The amdS gene mediated transformation with a 140-fold higher is the only exception and it confers the ability to transformation frequency than protoplast utilize acetamide as the sole carbon/nitrogen source in 23 transformation was adjudged the best21. The ease of a wild type background . Table 1 lists the nutritional the protocol coupled with the wide host range of selectable markers employed