§ 90. Commissioner and deputy commissioner of public works. The commissioner of public works shall be the head of the department of public works. He shall appoint, to hold office during his pleasure, a deputy and such other subordinates as may be prescribed by the board of estimate and apportionment. In case of the absence or disability of the commissioner or of a vacancy in the office, the deputy commissioner shall discharge the duties of the office until the commissioner returns, his disability ceases or the vacancy is filled. The commissioner and deputy commissioner before entering upon the discharge of the duties of their respective offices shall each execute and file with the city clerk an official undertaking in such penal sum as may be prescribed by the common council.