Ray J M, Bhaya D, Block M A, Grossman A R
Department of Plant Biology, Carnegie Institution of Washington, Stanford, California 94305.
J Bacteriol. 1991 Jul;173(14):4297-309. doi: 10.1128/jb.173.14.4297-4309.1991.
The alkaline phosphatase of Synechococcus sp. strain PCC 7942 is 145 kDa, which is larger than any alkaline phosphatase previously characterized and approximately three times the size of the analogous enzyme in Escherichia coli. The gene for the alkaline phosphatase, phoA, was cloned and sequenced, and the protein that it encodes was found to have little similarity to other phosphatases. Some sequence similarities were observed between the Synechococcus sp. strain PCC 7942 alkaline phosphatase, the alpha subunit of the ATPase from bacteria and chloroplasts, and the UshA sugar hydrolase of E. coli. Also, limited sequence similarity was observed between a region of the phosphatase and a motif implicated in nucleotide binding. Interestingly, although the alkaline phosphatase is transported across the inner cytoplasmic membrane and into the periplasmic space, it does not appear to have a cleavable signal sequence at its amino terminus. The half-life of the mRNA encoding the alkaline phosphatase, measured after inhibition of RNA synthesis, is approximately 5 min. Similar kinetics for the loss of alkaline phosphatase mRNA occur upon the addition of phosphate to phosphate-depleted cultures, suggesting that high levels of this nutrient inhibit transcription from phoA almost immediately. The phoA gene also appears to be the first gene of an operon; the largest detectable transcript that hybridizes to a phoA gene-specific probe is 11 kb, over twice the size needed to encode the mature protein. Other phosphate-regulated mRNAs are also transcribed upstream of the phoA gene. Insertional inactivation of phoA results in the loss of extracellular, phosphate-regulated phosphatase activity but does not alter the capacity of the cell for phosphate uptake.
聚球藻属PCC 7942菌株的碱性磷酸酶为145 kDa，比之前鉴定的任何碱性磷酸酶都要大，约为大肠杆菌中类似酶大小的三倍。碱性磷酸酶基因phoA被克隆并测序，发现其编码的蛋白质与其他磷酸酶几乎没有相似性。在聚球藻属PCC 7942菌株的碱性磷酸酶、细菌和叶绿体ATP酶的α亚基以及大肠杆菌的UshA糖水解酶之间观察到一些序列相似性。此外，在磷酸酶的一个区域与一个涉及核苷酸结合的基序之间观察到有限的序列相似性。有趣的是，尽管碱性磷酸酶被转运穿过内膜进入周质空间，但它在氨基末端似乎没有可切割的信号序列。在抑制RNA合成后测定的编码碱性磷酸酶的mRNA半衰期约为5分钟。在向缺磷培养物中添加磷酸盐后，碱性磷酸酶mRNA的丢失也出现类似的动力学，这表明这种营养物质的高水平几乎立即抑制phoA的转录。phoA基因似乎也是一个操纵子的第一个基因；与phoA基因特异性探针杂交的最大可检测转录本为11 kb，是编码成熟蛋白所需大小的两倍多。其他受磷酸盐调节的mRNA也在phoA基因上游转录。phoA的插入失活导致细胞外磷酸盐调节的磷酸酶活性丧失，但不改变细胞吸收磷酸盐的能力。
