Identification of cold shock gene loci in Sinorhizobium meliloti by using a luxAB reporter transposon

Using a luxAB reporter transposon, seven mutants of Sinorhizobium meliloti, were identified as containing insertions in four cold shock loci. LuxAB ac tivity was strongly induced (25- to 160-fold) after a temperature shift fro m 30 to 15 degrees C, The transposon and flanking host DNA from each mutant was cloned, and the nucleic acid sequence of the insertion site was determ ined. Unexpectedly, five of the seven luxAB mutants contained transposon in sertions in the 16S and 23S rRNA genes of two of the three nn operons of S. meliloti. Directed insertion of LuxAB genes into each of the three rrn ope rons revealed that all three operons were similarly affected by cold shock. Two other insertions were found to be located downstream of a homolog of t he major Escherichia coli cold shock gene, cspA, Although the cold shock lo ci were highly induced in response to a shift to low temperature, none of t he insertions resulted in a statistically significant decrease in growth ra te at 15 degrees C.