Port Elizabeth - The current impasse between coalition partners the DA and the UDM in Nelson Mandela Bay has taken a new turn, with the UDM calling for executive mayor Athol Trollip's replacement.DA leader Mmusi Maimane wrote to UDM leader Bantu Holomisa on Thursday urging him not to hold the coalition to ransom amid reports of alleged corruption that led to the ousting of the UDM's deputy mayor Mongameli Bobani.- Read more: Maimane urges UDM to stick with Nelson Mandela Bay coalitionHolomisa had previously said his party would be pulling out of the coalition agreement unless Bobani, who was voted out on Thursday during a motion of no confidence, is reinstated by Monday.The UDM leader has since responded to Maimane's letter in an email, threatening the coalition further by saying they would consider working with the DA again if they replaced Trollip as mayor.Coalition agreement"If you want UDM to continue to participate in the NMBMM coalition we would suggest that you replace Trollip with someone who would respect the coalition agreement," Holomisa wrote to the parties on Friday."His unilateral decision to sneak Patriotic Alliance through the back door without the knowledge of other coalition partners smacks of arrogance," he said, referring to PA member Marlon Daniels sponsoring the motion of no confidence that saw Bobani voted out.Holomisa also claimed what he termed Trollip's "defiance" of coalition leadership decisions undermined the UDM, and "raises some serious questions as to who is in charge of the DA"."If we take decisions as leaders together with you and are later vetoed by Trollip you can't expect us to be stooges in this coalition."He requested the DA provide the evidence it has of Bobani's reported corruption compiled by auditing firm PriceWaterhouseCoopers.'Put Nelson Mandela Bay first'The DA on Friday continued to urge Holomisa and the UDM to put the people of Nelson Mandela Bay first.DA federal council chairperson James Selfe said in a statement that voters chose to remove the ANC government in the city, and a collapse of the coalition would fail them."The DA urges Mr Holomisa to now put the people of NMB first. These women and men voted to remove the ANC and replaced it with the coalition government."It is crucial that we do not let them down. Mr Holomisa must not prioritise Mr Bobani at the expense of the millions who have put their faith in the coalition government."Selfe said that the Bobani saga was nine months in the making, and the party could not tolerate no action by the UDM any longer."The reality is that the NMB coalition government has achieved much since being elected, in no small part due to our other coalition partners who have remained committed to our coalition agreement to zero tolerance for corruption."However, former deputy mayor Bobani has been a destructive member of this particular coalition, and his actions have served to undermine service delivery, most especially to the poorest and most vulnerable communities."Council majority under threatSelfe said Bobani voting with the ANC in the council was unacceptable and contrary to the coalition agreement.Bobani still needed to answer serious allegations of irregular expenditure, fraud and corruption that allegedly took place in the directorate for which he was previously responsible, he said.In spite of this, the DA remained committed to working with the UDM and its other coalition partners to deliver corruption-free and efficient government, Selfe added.The coalition in the metro also includes the African Christian Democratic Party, the Congress of the People and the Freedom Front Plus.The coalition parties together hold 61 of the 120 seats in the council.The UDM's withdrawal of its two seats would leave the coalition a seat shy of a 50% + 1 majority, threatening its ability to pass motions.