Abstract: A method for identifying an essential gene of an organism comprises: (i) providing a Library of transposon mutants of the said organism; (ii) isolating polynucleotide sequences from the library which flank inserted transposons; (iii) hybridising the said polynucleotide sequences with a polynucleotide library from the said organism; and (iv) identifying a polynucleotide in the said polynucleotide library to which the said polynucleotide sequences do not hybridise, thereby to determine an essential gene of the organism.

Abstract: A transposon which comprises an RNA polymerase recognition site and a homing endonuclease recognition site. The transposon may be used in methods for the identification of an essential gene or a conditional essential gene of an organism. Genes identified in such methods are useful as substrates for use in screening for antibacterials, antiparasitics, fungicides, pesticides and herbicides.