Cointegration of transforming DNAs in Aspergillus nidulans: a model using autonomously-replicating plasmids

Co-transformation with autonomous replicating and integrative plasmids in Penicillium chrysogenum is highly efficient and leads in some cases to rescue of the intact integrative plasmid

The efficiency of co-transformation in Penicillium chrysogenum Wisconsin 54-1255 pyrG(-) and the fate of the transforming DNA were studied using an integrative (pEF43) and an autonomous replicating plasmid (pAM9L). The results showed a co-transformation frequency of nearly 70% of all transformants tested. The total efficiency of transformation was shown to be dependent on the plasmid marker used as transformant selection (i.e., markers in the integrative or autonomous replicating vector). Analysis of the plasmids re-isolated from several co-transformants showed that different populations of plasmids co-exist in the fungal host. Interestingly, in all co-transformants studied, the integrative plasmid was found to be replicating autonomously without integrating into the host genome. In some cases, co-integrates were formed by recombination between autonomous replicating (pAM9L) and integrative (pEF43) plasmids. However, unexpectedly in some cases, the non-reorganised pEF43 integrative plasmid used in the co-transformation assays was rescued from some co-transformants.

Molecular characterization of transforming plasmid rearrangements in transgenic rice reveals a recombination hotspot in the CaMV 35S promoter and confirms the predominance of microhomology mediated recombination

The characterization of plasmid-genomic DNA junctions following plant transformation has established links between DNA double-strand break repair (DSBR), illegitimate recombination and plasmid DNA integration. The limited information on plasmid-plasmid junctions in plants comes from the dicot species tobacco and Arabidopsis. We analyzed 12 representative transgenic rice lines, carrying a range of transforming plasmid rearrangements, which predominantly reflected microhomology mediated illegitimate recombination involving short complementary patches at the recombining ends. Direct end-ligation, in the absence of homology between the recombining molecules, occurred only rarely. Filler DNA was found at some of the junctions. Short, purine-rich tracts were present, either at the junction site or in the immediate flanking regions. Putative DNA topoisomerase I binding sites were clustered around the junctions. Although different regions of the transforming plasmid were involved in plasmid-plasmid recombination, we showed that a 19 bp palindromic sequence, including the TATA box of the CaMV 35S promoter, acted as a recombination hotspot. The purine-rich half of the palindromic sequence was specifically involved at the recombination junctions. This recombination hotspot is located within the 'highly recombinogenic' region of the full-length CaMV RNA that has been shown to promote viral recombination in dicot plants. Clustering of plasmid recombination events in this highly recombinogenic region, even in the absence of viral enzymes and other cis-acting elements proves that the plant cellular machinery alone is sufficient to recognize and act on these viral sequences. Our data also show the similarity between mechanisms underlying junction formation in dicot and monocot plants transformed using different procedures.

Autonomous plasmid replication in Aspergillus nidulans: AMA1 and MATE elements

With few exceptions, in eukaryotic organisms the presence of a chromosomal replicator on a circular vector molecule is not sufficient to confer on it the ability to persist and replicate extrachromosomally. However, it is possible to isolate from genomes of some filamentous fungi DNA fragments which can provide extrachromosomal maintenance of plasmids. In Aspergillus nidulans, two functional classes of such sequences can be distinguished: effective plasmid replicators (e.g., AMA1) and transformation enhancers (e.g., ANS1 or MATEs), which apparently are able to initiate aberrant replication, leading to vector rearrangement and multimerization and eventually resulting in chromosomal integration. We discuss the similarity of these events to DNA amplification in other eukaryotes. A model is suggested which accounts for the formation of effective replicating plasmids as a result of sequence amplification. The model is based on the observation that in some organisms, including A. nidulans and Schizosaccharomyces pombe, duplication of an inefficient replicator enhances its efficiency dramatically. Some structural traits of transformation enhancers in A. nidulans imply a role for topoisomerases in amplification and replication of circular DNA molecules. We discuss practical applications of replicative vectors for gene cloning and expression studies.

Mechanisms of fungicide resistance in phytopathogenic fungi

The disciplines traditionally used to investigate the mode of action of fungicides have been biochemistry and physiology. Over the past decade, classical and molecular genetics have been brought to bear on this problem with increasing success. Recently, genetic studies of fungicide resistance have led to advances in our understanding of the site of action of chemicals active against plant pathogens and, in some cases, to an appreciation of additional mechanisms of resistance to fungicide action.

Multiple copies of MATE elements support autonomous plasmid replication in Aspergillus nidulans

The AMA1 sequence is an efficient plasmid replicator and transformation enhancer in Aspergillus nidulans. It comprises two long perfect inverted repeats (MATE elements) flanking a short, unique, central spacer. Subclone analysis indicates that the complete inverted duplication, but not the unique central spacer, is necessary for efficient plasmid replication. The smallest fragments able to affect transformation efficiency lie within the AT-rich portions of the inverted repeats. We demonstrate that two or more copies of the repeat in any relative orientation are able to perform the replicator function. A single copy of a MATE element increases transformation frequency to a modest extent but leads to multiple rearrangement, unstable integration or concatenation of vector molecules. Multimeric concatenates generated during this process are more sable mitotically, and when reisolated, transform the fungus at a much higher frequency than the original monomeric vector. Selection for multiple products which resemble amplified DNA in various eukaryotic systems.

Integrative and replicative transformation of Penicillium canescens with a heterologous nitrate-reductase gene

A wild isolate of Penicillium canescens was subjected to mutagenesis, and 150 chlorate-resistant mutants were isolated and classified in respect of their ability to utilize various nitrogen sources. Strains supposedly deficient in nitrate reductase have been transformed with the nitrate-reductase gene from Aspergillus niger. Transformation probably occurred by non-homologous integration of the transforming vector into the chromosome. Co-transformation with the AMA 1 replicating element from A. nidulans enhanced transformation frequency up to 2000-fold, and was shown to result in autonomous maintenance of replicating concatenates, one of which was re-isolated by transformation of E. coli.

Recombinational stability of replicating plasmids in Aspergillus nidulans during transformation, vegetative growth and sexual reproduction

Plasmids containing the AMA1 replicon are capable of autonomous maintenance in Aspergillus nidulans. It has been reported previously that these plasmids can form concatenates by recombination in a transformed mycelium, and up to 10% of molecules are involved in such events. The present study demonstrates that plasmid recombination, although frequent during transformation, rarely occurs during vegetative growth. As a result, the structure and phenotypic stability of AMA1 plasmids generally remains unaltered for many asexual (conidial) generations. It is also evident that plasmid replication does not require specific recombination events in the AMA1 palindrome. However, during sexual reproduction, autonomous plasmids exhibit increased recombination, which results in both plasmid concatenation and integration into the chromosome.