NOVELTY - A non-natural derivative of a naturally-occurring riboswitch, is new. The naturally-occurring riboswitch is a glutamine-responsive riboswitch, S-adenosylmethionine (SAM)-responsive riboswitch, S-adenosylhomocysteine (SAH)-responsive riboswitch, glutamine riboswitch, SAM/SAH riboswitch, glnA riboswitch, Downstream-peptide riboswitch, crcB riboswitch, pfl riboswitch, yjdF riboswitch, manA riboswitch, wcaG riboswitch, epsC riboswitch, ykkC-III riboswitch, psaA riboswitch, psbA riboswitch, PhotoRC-I riboswitch, PhotoRC-II riboswitch, or psbNH riboswitch.    USE - For detecting or identifying compounds, useful for killing or inhibiting the growth of bacterial cell (claimed). The bacteria are e.g. Bacillus anthracis, Enterococcus facecalis and Escherichia coli. Also useful for dental treatments and purification of water (e.g. municipal water, sewage treatment systems, potable and non-potable water supplies and hatcheries).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) new regulatable gene expression construct comprising: a nucleic acid molecule encoding an RNA comprising the riboswitch operably linked to a coding region, where the riboswitch regulates expression of the RNA, and the riboswitch and coding region are heterologous;    (2) a method (M1) of detecting a compound of interest, involving: bringing into contact a sample and the riboswitch, where the riboswitch is activated by the compound of interest, and the riboswitch produces a signal when activated by the compound of interest and the riboswitch produces a signal when the sample contains the compound of interest;    (3) a method involving: a) testing a compound for altering gene expression of a gene encoding an RNA comprising the riboswitch, where the alteration is via the riboswitch; and altering gene expression by bringing into contact a cell and a compound that altered gene expression in step (a), where the cell comprises a gene encoding an RNA comprising a riboswitch, where the compound inhibits expression of the gene by binding to the riboswitch;    (4) a method of identifying riboswitches, involving: assessing in-line spontaneous cleavage of RNA molecule in the presence and absence of a compound, where the RNA molecule is encoded by a gene regulated by the compound, and a change in the pattern of in-line spontaneous cleavage of the RNA molecule indicates a riboswitch, and a) the RNA comprises a glutamine-responsive riboswitch or a derivative of a glutamine-responsive riboswitch and the compound is glutamine, b) the RNA comprises SAH-responsive riboswitch or its derivative and the compound is SAH, or c) the RNA comprises SAM-responsive riboswitch or its derivative and the compound is SAM; and    (5) a method (M2) of altering gene expression, involving bringing into contact a compound and a cell, where the cell comprises a gene encoding the RNA.