The transcription levels of argB-20 and argB-37 at 20uC and 37uC were quantified by real time RT-PCR. Results showed that the mRNA level of argB-20 at 20uC was significantly higher that at 37uC and the mRNA level of argB-37 at 37uC was significantly higher that at 20uC ( Figure 4C and 4D), suggesting that their expressions, similar to most other bacterial genes, were controlled at the transcription level. When argB-20 and argB-37 were cloned, expressed and the corresponding proteins NAGK-20 and NAGK-37 purified for enzyme assays, their highest enzymatic activities were observed at 37-45uC and 45-50uC respectively ( Figure 4E) . Moreover, NAGK-20, but not NAGK-37, was inhibited by 0.25-10 mM of arginine ( Figure 4F ).