Gene expression is a fundamental process in all life forms. In bacterial cells, for example, gene expression is known to be tightly regulated in response to environmental changes. Even in the same environment, certain genes are expressed with great cell-to-cell variation, resulting in phenotypic heterogeneity within clonal populations. My research focuses on understanding how gene expression is turned on and off in fluctuating environments and how population-wide heterogeneity is established under a given condition. Previous studies have shown that the DNA-binding and -unbinding kinetics of a transcription factor, along with the associated kinetics of transcription initiation, make major contributions to these two questions. However, gene expression involves both initiation and elongation by molecular motors (RNA polymerase and ribosomes), and the latter elongation dynamics may also be important in gene regulation. I will present the results of in vivo and in silico studies illustrating that the dynamics of elongation by multiple RNA polymerases on a single DNA template play a considerable role in regulating many aspects of gene expression.