Isothermal titration calorimetry (ITC) is the method of choice for obtaining thermodynamic data on a great variety of systems. Here we show that modern ITC apparatus and new processing methods allow researchers to obtain a complete kinetic description of systems more diverse than previously thought, ranging from simple ligand binding to complex RNA folding. We illustrate these new features with a simple case (HIV-1 reverse transcriptase/inhibitor interaction) and with the more complex case of the folding of a riboswitch triggered by the binding of its ligand. The originality of the new kinITC method lies in its ability to dissect, both thermodynamically and kinetically, the two components: primary ligand binding and subsequent RNA folding. We are not aware of another single method that can yield, in a simple way, such deep insight into a composite process. Our study also rationalizes common observations from daily ITC use.

The skeletal structures of the face and throat are derived from cranial neural crest cells (NCCs) that migrate from the embryonic neural tube into a series of branchial arches (BAs). The first arch (BA1) gives rise to the upper and lower jaw cartilages, whereas hyoid structures are generated from the second arch (BA2). The Hox paralogue group 2 (PG2) genes, Hoxa2 and Hoxb2, show distinct roles for hyoid patterning in tetrapods and fishes. In the mouse, Hoxa2 acts as a selector of hyoid identity, while its paralogue Hoxb2 is not required. On the contrary, in zebrafish Hoxa2 and Hoxb2 are functionally redundant for hyoid arch patterning. Here, we show that in Xenopus embryos morpholino-induced functional knockdown of Hoxa2 is sufficient to induce homeotic changes of the second arch cartilage. Moreover, Hoxb2 is downregulated in the BA2 of Xenopus embryos, even though initially expressed in second arch NCCs, similar to mouse and unlike in zebrafish. Finally, Xbap, a gene involved in jaw joint formation, is selectively upregulated in the BA2 of Hoxa2 knocked-down frog embryos, supporting a hyoid to mandibular change of NCC identity. Thus, in Xenopus Hoxa2 does not act redundantly with Hoxb2 for BA2 patterning, similar to mouse and unlike in fish. These data bring novel insights into the regulation of Hox PG2 genes and hyoid patterning in vertebrate evolution and suggest that Hoxa2 function is required at late stages of BA2 development. Developmental Dynamics 234:858 -867, 2005.