ABSTRACT
Gene enrichment selects for differentially expressed genes using techniques such as differential expression analysis and gene targeting. Downstream screening approaches can be activity-based through the screening of expression libraries, sequence-dependent by using gene targeting or can be sequence-independent through the direct sequencing of the metagenome. Reverse transcriptase polymerase chain reaction (RT-PCR) has been used to recover genes from environmental samples, for example in the isolation of naphthalene-degrading enzymes from microorganisms present in a coal tar waste. Integrons therefore act as a repository of open reading frames coding for many gene products and potentially provide a source of novel genes. Microarrays of immobilized oligonucleotide gene targets have also been used to select appropriate biotope samples for metagenomic library screening. In addition, the process of RT-PCR amplification limits the size of inserts and could impose a large sequence-dependent bias on the library. Gene-specific PCR has been used extensively to probe communities for microorganisms with specific metabolic or biodegradative capabilities.
