Leedjärv Anu, Ivask Angela, Virta Marko
Department of Applied Chemistry and Microbiology, University of Helsinki, P.O. Box 56, FIN-00014 Helsinki, Finland.
J Bacteriol. 2008 Apr;190(8):2680-9. doi: 10.1128/JB.01494-07. Epub 2007 Dec 7.
According to in silico analysis, the genome of Pseudomonas putida KT2440 encodes at least four Zn/Cd/Pb efflux transporters-two P-type ATPases (CadA1 and CadA2) and two czc chemiosmotic transporters (CzcCBA1 and CzcCBA2). In this study we showed that all these transporters are functional, but under laboratory conditions only two of them were involved in the mediation of heavy metal resistance in P. putida KT2440. CadA2 conferred Cd(2+) and Pb(2+) resistance, whereas CzcCBA1 was involved in export of Zn(2+), Cd(2+), and possibly Pb(2+). CadA1, although nonfunctional in P. putida, improved Zn(2+) resistance and slightly improved Cd(2+) resistance when it was expressed in Escherichia coli. CzcCBA2 contributed to Zn resistance of a czcA1-defective P. putida strain or when the CzcA2 subunit was overexpressed in a transporter-deficient strain. It seemed that CzcA2 could complex with CzcC1 and CzcB1 subunits and therefore complement the loss of CzcA1. The CzcCBA2 transporter itself, however, did not function. Expression of cadA1, cadA2, and czcCBA1 was induced by heavy metals, and the expression levels were dependent on the growth medium and growth phase. Expression of cadA2 and czcCBA1 was nonspecific; both genes were induced by Zn(2+), Cd(2+), Pb(2+), Ni(2+), Co(2+), and Hg(2+). On the other hand, remarkably, expression of cadA1 was induced only by Zn(2+). Possible roles of distinct but simultaneously functioning transporters are discussed.
根据计算机模拟分析，恶臭假单胞菌KT2440的基因组编码至少四种锌/镉/铅外排转运蛋白——两种P型ATP酶（CadA1和CadA2）以及两种czc化学渗透转运蛋白（CzcCBA1和CzcCBA2）。在本研究中，我们表明所有这些转运蛋白均具有功能，但在实验室条件下，其中只有两种参与了恶臭假单胞菌KT2440对重金属的抗性调节。CadA2赋予了对Cd(2+)和Pb(2+)的抗性，而CzcCBA1参与了Zn(2+)、Cd(2+)以及可能的Pb(2+)的外排。CadA1虽然在恶臭假单胞菌中无功能，但当它在大肠杆菌中表达时，可提高对Zn(2+)的抗性，并略微提高对Cd(2+)的抗性。CzcCBA2有助于czcA1缺陷型恶臭假单胞菌菌株对锌的抗性，或者当CzcA2亚基在转运蛋白缺陷型菌株中过表达时也有此作用。似乎CzcA2可以与CzcC1和CzcB1亚基形成复合物，从而弥补CzcA1的缺失。然而，CzcCBA2转运蛋白本身并无功能。cadA1、cadA2和czcCBA1的表达受重金属诱导，且表达水平取决于生长培养基和生长阶段。cadA2和czcCBA1的表达是非特异性的；这两个基因均受Zn(2+)、Cd(2+)、Pb(2+)、Ni(2+)、Co(2+)和Hg(2+)诱导。另一方面，值得注意的是，cadA1的表达仅受Zn(2+)诱导。本文讨论了不同但同时发挥作用的转运蛋白的可能作用。
